ESTMP Keywords and Verbs (commands) Defined - Fluffy the SMTPGuardDog - spam and virus filter for any SMTP server

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Fluffy is an SMTP proxy - that is it relays commands between two SMTP servers, altering and blocking commands and responses to block junk mail and other undesirable mail. Fluffy needs an understanding of the different SMTP commands so that it can track the state of the SMTP server. When the SMTP server is receiving a mail message we don't want Fluffy to consider the data stream to be commands.

ESMTP is an extension to SMTP which allows additional commands to be defined. If Fluffy doesn't recognise a command it will pass on that command on to the local SMTP server (and echo its response back). This works for any command that doesn't change the state of the SMTP server (say from the processing command state to the accepting mail state). But Fluffy will still throw up warnings.

My aim is to have Fluffy recognise how to handle the various ESMTP commands it might receive. This started me on a quest to identify the commands and figure out how Fluffy should handle them. I didn't find a complete source of information, and met other people who had partially completed the same quest or were trying to understand just one command.

So here I am documenting what I have learnt, in the hope that others might find it useful. Please let me know any errors or additions

Thanks for corrections and additions to:

Michael J. Ashcroft
Bruce Gingery

Under ESMTP, the EHLO greeting will trigger a response which will list ESMTP keywords. Basically this details what features the SMTP mail server supports. Such features may also have 'verbs' (or commands) associated with them that are used during the SMTP conversation.

Keywords/Verbs that beging with the letter X are proprietary extensions.

Token Type Reference SMTP server Comments

8BITMIME Keyword RFC1652 Most Indicates support for 8 bit encoded (rather than 7 bit) mail message contents, through the optional addition of a parameter to the MAIL command

ATRN Keyword RFC2645 Some Indicates support for the ATRN verb. Only valid in an on-demand SMTP server running on port 366

ATRN Verb RFC2645 Some Operates as an authorised TURN equivalent for SMTP servers with a dynamic IP address

AUTH Keyword RFC2554 Most Indicates support for the AUTH verb

AUTH Verb RFC2554 Most Authentication. If accepted challenge/response pairs are sent 1 or more times

AUTH= Keyword RFC2554 Some This keyword is not valid, but seems to indicate support for the AUTH parameter with the MAIL verb. That support should be indicated with the AUTH keyword

BDAT Verb RFC3030 Microsoft Exchange Server Should only be used if the CHUNKING keyword is sent. Like the DATA command, but if accepted data of a length specified in BDAT follows. No need for a terminator. Can be multiple chunks - the last BDAT command comes with a LAST parameter. Cannot mix BDAT and DATA

BINARYMIME Keyword RFC3030 Microsoft Exchange Server Indicates support for binary encoded (rather than 7 bit) mail message contents, through the optional addition of a parameter to the MAIL command.

CHECKPOINT Keyword RFC1845 Some Indicates support for restarting an interrupted SMTP conversation by adding an optional parameter to the MAIL command

CHUNKING Keyword RFC3030 Microsoft Exchange Server Indicates support for the BDAT command

CONNEG Keyword SMTP and MIME Extensions For Content Conversion Indicates support for the CONNEG parameter to the RCPT-TO command

CONPERM Keyword SMTP and MIME Extensions For Content Conversion Indicates support for the CONPERM paramter for the MAIL FROM command

DATA Verb RFC2821 Required If accepted, means that subsequent text makes up a mail message, until a terminator (CR LF . CR LF) is received

DEBUG Verb Sendmail,Zmailer If accepted, puts receiving SMTP server into debug mode

DELIVERBY Keyword RFC2852 Most Indicates support for not delivering mail after a designated time. Comes with reporting paramters and adds an optional parameter to the MAIL command

DSN Keyword RFC1891 Most Indicates support for Delivery Status Notification by adding optional parameters to the MAIL and RCPT command

EHLO Keyword CommuniGate Pro Some faulty SMTP servers will incorrectly list EHLO as a keyword

EHLO Verb RFC1869 Most Alternative to HELO. Returns list of keywords identifying SMTP extensions supported by receiving SMTP server

EMAL Verb IETF Archives Zmailer Start of EMAL FROM: Like MAIL except planned to indicate (or allow) 8 bit support for the data contents

ESAM Verb IETF Archives Zmailer Start of ESAM FROM: Like SAML except planned to indicate (or allow) 8 bit support for the data contents

ESND Verb IETF Archives Zmailer Start of ESND FROM: Like SEND except planned to indicate (or allow) 8 bit support for the data contents

ESOM Verb IETF Archives Zmailer Start of ESOM FROM: Like SOML except planned to indicate (or allow) 8 bit support for the data contents

ENHANCEDSTATUSCODES Keyword RFC2034 Most Indicates responses will include an enhanced status code

ENHANGEDSTATUSCODES Keyword RFC2034 Zmailer - Obsolete A typo for enhanCed. Indicates responses will include an enhanced status code

ETRN Keyword RFC1985 Some Indicates support for the ETRN verb

ETRN Verb RFC1985 Some Requests the local SMTP server to initiate delivery of mail to the external SMTP server on a separate SMTP connection

EVFY Verb IETF Archives Zmailer Planned to return whether an address is likely to accept 8 bit messages

EXDATA Keyword RFC2821 Courier Indicates support for the EXDATA parameter with the MAIL FROM command

EXPN Keyword RFC2821 Many Indicates support for the EXPN verb

EXPN Verb RFC2821 Many Expand a mailing list address into individual recipients. Often disabled to prevent use by spammers

HELO Verb RFC2821 Required Should be the first command is an SMTP conversation. Identifies sending SMTP server

HELP Keyword RFC2821 Many Indicates support for the HELP verb

HELP Verb RFC2821 Many Returns help message

IDENT Verb RFC1431 Zmailer Returns the response from an IDENT query (preusmably on your own connection)

LANG Keyword SMTP Language Extension Specify language for response messages

LANGUAGE Keyword SMTP Language Extension Indicates support for the LANG verb

MAIL Verb RFC2821 Required Start of MAIL FROM: Identifies sender of mail message. May be forged. May not correspond to the From: line in a mail message. Should be added in Return Path header. Address to send any undeliverable notifications (bounces)

NOOP Verb RFC2821 Required NO OPeration. Does nothing but keeps the connection active and can help synchronise commands and responses.

OK Keyword Microsoft Exchange Server Some faulty SMTP servers will incorrectly list this as a keyword

ONEX Keyword Sendmail - Obsolete Indicates support for the ONEX verb

ONEX Verb Sendmail - Obsolete Indicates that only one mail message is coming in this connection - stops Sendmail from forking a new process (supposed to make it more efficient)

PIPELINING Keyword RFC2920 Most Indicates support for a command stream without waiting for a response to each command. The receiving server is supposed to batch responses. Streaming is supposed to stop with a command that can change the state of the receiving SMTP server (such as the DATA command)

QUIT Verb RFC2821 Required Disconnects

RCPT Verb RFC2821 Required Start of RCPT TO: Identifies recipient of mail message.

READY Keyword FTGate Some faulty SMTP servers will incorrectly list this as a keyword

RSET Verb RFC2821 Required Resets the server state

SAML Keyword RFC2821 Obsolete Indicates support for the SAML verb

SAML Verb RFC2821 Obsolete Start of SAML FROM: Like MAIL except supposed to also display the message on the recipients computer (early form of instant messaging)

SOML Keyword RFC2821 Obsolete Indicates support for the SOML verb

SOML Verb RFC2821 Obsolete Start of SAML FROM: Like MAIL except supposed to either mail the message OR display the message on the recipients computer (early form of instant messaging)

SEND Keyword RFC2821 Obsolete Indicates support for the SEND verb

SEND Verb RFC2821 Obsolete Start of SEND FROM: Like MAIL except supposed to display the message on the recipients computer (early form of instant messaging)

SIZE Keyword RFC1870 Most Indicates the maximum size of messages that will be accepted, and indicates support for message sizes to be included as a parameter with the MAIL verb

QUEU Verb IBM VM SMTP Returns mail delivery queue information

RELAY Keyword Draft RFC 2476 PMDF - Obsolete Part of a draft which was dropped in the final version

STARTTLS Keyword RFC2487 Some Indicates support for the STARTTLS verb.

STARTTLS Verb RFC2487 Some Triggers start of TLS negotiation for secure SMTP conversation. If successful, resets state to before EHLO command sent

SUBMITTER Keyword Responsible Submitter of an E-mail Message Indicates support for the SUBMITTER Parameter of the MAIL command

TETHYS Keyword FTGate Unknown

TICK Keyword PMDF - Obsolete Indicates support for the TICK verb. There were only ever used in BSMTP (Batch SMTP).

TICK Verb IBM VM SMTP, PMDF - Obsolete Returns a ticket number for BSMTP

TIME Keyword Mercury Indicates support for the TIME verb.td>

TIME Verb Mercury Returns the date and time of the receiving mail server

TLS Keyword Draft of RFC2487 Microsoft Exchange Server - Onsolete Indicates support for the TLS verb.

TLS Verb Draft of RFC2487 Microsoft Exchange Server - Obsolete Triggers start of TLS negotiation for secure SMTP conversation. If successful, resets state to before EHLO command sent

TURN Verb RFC2821 Obsolete If accepted swaps the sending/receveing roles of the external/local SMTP servers. Not recommended due to lack of authentication

TURNME Verb Zmailer - Obsolete Equivalent to ETRN

VERB Keyword Sendmail Indicates support for the VERB verb

VERB Verb Sendmail Enableds verbose (detailed) responses

VRFY Verb RFC2821 Required Returns whether specified email address is a valid recipient. Often disabled to prevent spammers testing a dictionary list of names to build up a list of valid recipients.

WITHELD Keyword FTGate Unknown - possibly indicates the host name is supressed?

X-ADAT Keyword ESMTP Service Extension for Filtering Gateways Indicates support for the X-ADAT verb

X-ADAT Verb ESMTP Service Extension for Filtering Gateways Allows sending of mail message before specifying recipients

X-DRCP Verb ESMTP Service Extension for Filtering Gateways Start of X-DRCPT TO: Allows deferred specifying recipients following use of X-ADAT

X-ERCP Verb ESMTP Service Extension for Filtering Gateways Specifies end of recipients following use of X-DRCP

X-EXPS Keyword Microsoft Microsoft Exchange Server Indicates support for the X-EXPS verb

X-EXPS Verb Microsoft Microsoft Exchange Server Microsoft version of AUTH. Seems to work the same way. If accepted challenge/response pairs are sent 1 or more times.

X=EXPS= Keyword Microsoft Exchange Server This seems to indicate support for the X-EXPS parameter with the MAIL verb.

X-LINK2STATE Keyword Microsoft Microsoft Exchange Server Indicates support for the X-LINK2STATE verb

X-LINK2STATE Verb Microsoft Microsoft Exchange Server Exchange of routing information. Supposedly generates one response

X-RCPTLIMIT Keyword Zmailer Parameter follows indicates maximum number of recipients per mail message

X-TURNME Keyword Zmailer - Obsolete Indicates support for the TURNME verb

XADR Keyword PMDF Indicates support for the XADR verb

XADR Verb PMDF Returns information about how an address is routed internally by PMDF as well as general channel information

XAUD Keyword NT Mail I'm guessing this is an audit ID reporting licence number and version

XAUTH Keyword Microsoft bug report Microsoft Exchange Server 5.0 Indicates support for the XAUTH verb

XAUTH Verb Microsoft bug report Microsoft Exchange Server 5.0 Authentication. If accepted challenge/response pairs are sent 1 or more times.

XBDATA Keyword Mailbeamer Details are unknown.

XCIR Keyword PMDF Indicates support for the XCIR verb

XCIR Verb PMDF Returns PMDF circuit check information

XDATASIZE Keyword LSMTP I'm guessing it's an obsolete form of XLSMTP-DATASIZE

XDSN Keyword RFC1891 Zmailer,Sendmail Indicates support for Delivery Status Notification by adding optional parameters to the MAIL and RCPT command

XEXCH50 Keyword Microsoft Microsoft Exchange Server Indicates support for the XEXCH50 verb

XEXCH50 Verb Microsoft Microsoft Exchange Server Allows transfer of binary data with Exchange specific recipient information (eg plain text only versus MIME, etc). If accepted, receiver SMTP servers sends 354 Send Binary data and sending SMTP server sends the number of bytes as the first parameter on the XEXCH50 command. Once these bytes are sent, the receiving SMTP server sends an acknowledgement

XGEN Keyword PMDF Indicates support for the XGEN verb

XGEN Verb PMDF Returns status information about whether a compiled configuration and compiled character set are in use

XLICENSE Verb LSMTP Returns license information

XLSMTP-ACKRCPT Keyword LSMTP I'm guessing it's linked with L-Soft's LISTSERV programme. There are probably associated keywords. Details are unknown.

XLSMTP-DATASIZE Keyword LSMTP I'm guessing it's linked with L-Soft's LISTSERV programme. There are probably associated keywords. Details are unknown.

XLSMTP-MMERGE Keyword LSMTP I'm guessing it's linked with L-Soft's LISTSERV programme and mail merge. There are probably associated keywords. Details are unknown.

XMBEAM30 Keyword Mailbeamer I'm guessing this simply identifies the software and version running. Details are unknown.

XONE Keyword Sendmail - Obsolete Indicates support for the ONEX verb

XQUE Keyword Sendmail - Obsolete Indicates support for the XQUE

XQUE Verb Sendmail - Obsolete I'm guessing this is equivalent to ETRN

XREMOTEQUEUE Keyword Netscape Mail server I'm guessing it indicates support for the XREMOTEQUEUE verb, to implement another version of the TURN command. Details are unknown.

XSBEAM30 Keyword Mailbeamer I'm guessing this simply identifies the software and version running. Details are unknown.

XSECURITY Keyword SECURITY SMTP Service Extension Courier Indicates support for the SECURITY parameter with the MAIL verb. This allows for specifying the secruity level for delivering mail.

XSTA Keyword PMDF Indicates support for the XSTA verb

XSTA Verb PMDF Returns status information about the number of messages processed and currently in the PMDF channel queues

XTRN Keyword SLMail Indicates support for the XTRN verb

XTRN Verb SLMail I believe this is equivalent to ETRN

XUSR Keyword Sendmail - Obsolete Indicates support for the XUSR verb

XUSR Verb Sendmail - Obsolete Indicates that the submitted mail is coming directly from the user, and not relayed from another SMTP server

XVERP Keyword Qmail,Postfix,Courier Indicates support for the XVERP parameter with the MAIL verb. This allows for automatic inclusion of the intended recipients address in the MAIL FROM address, so that bounces clearly show the address of the intended recipient.

XVRB Keyword Sendmail Indicates support for the VERB verb

XXWALL30 Keyword XWall I'm guessing this simply identifies the software and version running. Details are unknown.

Token	Type	Reference	SMTP server	Comments
8BITMIME	Keyword	RFC1652	Most	Indicates support for 8 bit encoded (rather than 7 bit) mail message contents, through the optional addition of a parameter to the MAIL command
ATRN	Keyword	RFC2645	Some	Indicates support for the ATRN verb. Only valid in an on-demand SMTP server running on port 366
ATRN	Verb	RFC2645	Some	Operates as an authorised TURN equivalent for SMTP servers with a dynamic IP address
AUTH	Keyword	RFC2554	Most	Indicates support for the AUTH verb
AUTH	Verb	RFC2554	Most	Authentication. If accepted challenge/response pairs are sent 1 or more times
AUTH=	Keyword	RFC2554	Some	This keyword is not valid, but seems to indicate support for the AUTH parameter with the MAIL verb. That support should be indicated with the AUTH keyword
BDAT	Verb	RFC3030	Microsoft Exchange Server	Should only be used if the CHUNKING keyword is sent. Like the DATA command, but if accepted data of a length specified in BDAT follows. No need for a terminator. Can be multiple chunks - the last BDAT command comes with a LAST parameter. Cannot mix BDAT and DATA
BINARYMIME	Keyword	RFC3030	Microsoft Exchange Server	Indicates support for binary encoded (rather than 7 bit) mail message contents, through the optional addition of a parameter to the MAIL command.
CHECKPOINT	Keyword	RFC1845	Some	Indicates support for restarting an interrupted SMTP conversation by adding an optional parameter to the MAIL command
CHUNKING	Keyword	RFC3030	Microsoft Exchange Server	Indicates support for the BDAT command
CONNEG	Keyword	SMTP and MIME Extensions For Content Conversion		Indicates support for the CONNEG parameter to the RCPT-TO command
CONPERM	Keyword	SMTP and MIME Extensions For Content Conversion		Indicates support for the CONPERM paramter for the MAIL FROM command
DATA	Verb	RFC2821	Required	If accepted, means that subsequent text makes up a mail message, until a terminator (CR LF . CR LF) is received
DEBUG	Verb		Sendmail,Zmailer	If accepted, puts receiving SMTP server into debug mode
DELIVERBY	Keyword	RFC2852	Most	Indicates support for not delivering mail after a designated time. Comes with reporting paramters and adds an optional parameter to the MAIL command
DSN	Keyword	RFC1891	Most	Indicates support for Delivery Status Notification by adding optional parameters to the MAIL and RCPT command
EHLO	Keyword		CommuniGate Pro	Some faulty SMTP servers will incorrectly list EHLO as a keyword
EHLO	Verb	RFC1869	Most	Alternative to HELO. Returns list of keywords identifying SMTP extensions supported by receiving SMTP server
EMAL	Verb	IETF Archives	Zmailer	Start of EMAL FROM: Like MAIL except planned to indicate (or allow) 8 bit support for the data contents
ESAM	Verb	IETF Archives	Zmailer	Start of ESAM FROM: Like SAML except planned to indicate (or allow) 8 bit support for the data contents
ESND	Verb	IETF Archives	Zmailer	Start of ESND FROM: Like SEND except planned to indicate (or allow) 8 bit support for the data contents
ESOM	Verb	IETF Archives	Zmailer	Start of ESOM FROM: Like SOML except planned to indicate (or allow) 8 bit support for the data contents
ENHANCEDSTATUSCODES	Keyword	RFC2034	Most	Indicates responses will include an enhanced status code
ENHANGEDSTATUSCODES	Keyword	RFC2034	Zmailer - Obsolete	A typo for enhanCed. Indicates responses will include an enhanced status code
ETRN	Keyword	RFC1985	Some	Indicates support for the ETRN verb
ETRN	Verb	RFC1985	Some	Requests the local SMTP server to initiate delivery of mail to the external SMTP server on a separate SMTP connection
EVFY	Verb	IETF Archives	Zmailer	Planned to return whether an address is likely to accept 8 bit messages
EXDATA	Keyword	RFC2821	Courier	Indicates support for the EXDATA parameter with the MAIL FROM command
EXPN	Keyword	RFC2821	Many	Indicates support for the EXPN verb
EXPN	Verb	RFC2821	Many	Expand a mailing list address into individual recipients. Often disabled to prevent use by spammers
HELO	Verb	RFC2821	Required	Should be the first command is an SMTP conversation. Identifies sending SMTP server
HELP	Keyword	RFC2821	Many	Indicates support for the HELP verb
HELP	Verb	RFC2821	Many	Returns help message
IDENT	Verb	RFC1431	Zmailer	Returns the response from an IDENT query (preusmably on your own connection)
LANG	Keyword	SMTP Language Extension		Specify language for response messages
LANGUAGE	Keyword	SMTP Language Extension		Indicates support for the LANG verb
MAIL	Verb	RFC2821	Required	Start of MAIL FROM: Identifies sender of mail message. May be forged. May not correspond to the From: line in a mail message. Should be added in Return Path header. Address to send any undeliverable notifications (bounces)
NOOP	Verb	RFC2821	Required	NO OPeration. Does nothing but keeps the connection active and can help synchronise commands and responses.
OK	Keyword		Microsoft Exchange Server	Some faulty SMTP servers will incorrectly list this as a keyword
ONEX	Keyword		Sendmail - Obsolete	Indicates support for the ONEX verb
ONEX	Verb		Sendmail - Obsolete	Indicates that only one mail message is coming in this connection - stops Sendmail from forking a new process (supposed to make it more efficient)
PIPELINING	Keyword	RFC2920	Most	Indicates support for a command stream without waiting for a response to each command. The receiving server is supposed to batch responses. Streaming is supposed to stop with a command that can change the state of the receiving SMTP server (such as the DATA command)
QUIT	Verb	RFC2821	Required	Disconnects
RCPT	Verb	RFC2821	Required	Start of RCPT TO: Identifies recipient of mail message.
READY	Keyword		FTGate	Some faulty SMTP servers will incorrectly list this as a keyword
RSET	Verb	RFC2821	Required	Resets the server state
SAML	Keyword	RFC2821	Obsolete	Indicates support for the SAML verb
SAML	Verb	RFC2821	Obsolete	Start of SAML FROM: Like MAIL except supposed to also display the message on the recipients computer (early form of instant messaging)
SOML	Keyword	RFC2821	Obsolete	Indicates support for the SOML verb
SOML	Verb	RFC2821	Obsolete	Start of SAML FROM: Like MAIL except supposed to either mail the message OR display the message on the recipients computer (early form of instant messaging)
SEND	Keyword	RFC2821	Obsolete	Indicates support for the SEND verb
SEND	Verb	RFC2821	Obsolete	Start of SEND FROM: Like MAIL except supposed to display the message on the recipients computer (early form of instant messaging)
SIZE	Keyword	RFC1870	Most	Indicates the maximum size of messages that will be accepted, and indicates support for message sizes to be included as a parameter with the MAIL verb
QUEU	Verb		IBM VM SMTP	Returns mail delivery queue information
RELAY	Keyword	Draft RFC 2476	PMDF - Obsolete	Part of a draft which was dropped in the final version
STARTTLS	Keyword	RFC2487	Some	Indicates support for the STARTTLS verb.
STARTTLS	Verb	RFC2487	Some	Triggers start of TLS negotiation for secure SMTP conversation. If successful, resets state to before EHLO command sent
SUBMITTER	Keyword	Responsible Submitter of an E-mail Message		Indicates support for the SUBMITTER Parameter of the MAIL command
TETHYS	Keyword		FTGate	Unknown
TICK	Keyword		PMDF - Obsolete	Indicates support for the TICK verb. There were only ever used in BSMTP (Batch SMTP).
TICK	Verb		IBM VM SMTP, PMDF - Obsolete	Returns a ticket number for BSMTP
TIME	Keyword		Mercury	Indicates support for the TIME verb.td>
TIME	Verb		Mercury	Returns the date and time of the receiving mail server
TLS	Keyword	Draft of RFC2487	Microsoft Exchange Server - Onsolete	Indicates support for the TLS verb.
TLS	Verb	Draft of RFC2487	Microsoft Exchange Server - Obsolete	Triggers start of TLS negotiation for secure SMTP conversation. If successful, resets state to before EHLO command sent
TURN	Verb	RFC2821	Obsolete	If accepted swaps the sending/receveing roles of the external/local SMTP servers. Not recommended due to lack of authentication
TURNME	Verb		Zmailer - Obsolete	Equivalent to ETRN
VERB	Keyword		Sendmail	Indicates support for the VERB verb
VERB	Verb		Sendmail	Enableds verbose (detailed) responses
VRFY	Verb	RFC2821	Required	Returns whether specified email address is a valid recipient. Often disabled to prevent spammers testing a dictionary list of names to build up a list of valid recipients.
WITHELD	Keyword		FTGate	Unknown - possibly indicates the host name is supressed?
X-ADAT	Keyword	ESMTP Service Extension for Filtering Gateways		Indicates support for the X-ADAT verb
X-ADAT	Verb	ESMTP Service Extension for Filtering Gateways		Allows sending of mail message before specifying recipients
X-DRCP	Verb	ESMTP Service Extension for Filtering Gateways		Start of X-DRCPT TO: Allows deferred specifying recipients following use of X-ADAT
X-ERCP	Verb	ESMTP Service Extension for Filtering Gateways		Specifies end of recipients following use of X-DRCP
X-EXPS	Keyword	Microsoft	Microsoft Exchange Server	Indicates support for the X-EXPS verb
X-EXPS	Verb	Microsoft	Microsoft Exchange Server	Microsoft version of AUTH. Seems to work the same way. If accepted challenge/response pairs are sent 1 or more times.
X=EXPS=	Keyword		Microsoft Exchange Server	This seems to indicate support for the X-EXPS parameter with the MAIL verb.
X-LINK2STATE	Keyword	Microsoft	Microsoft Exchange Server	Indicates support for the X-LINK2STATE verb
X-LINK2STATE	Verb	Microsoft	Microsoft Exchange Server	Exchange of routing information. Supposedly generates one response
X-RCPTLIMIT	Keyword		Zmailer	Parameter follows indicates maximum number of recipients per mail message
X-TURNME	Keyword		Zmailer - Obsolete	Indicates support for the TURNME verb
XADR	Keyword		PMDF	Indicates support for the XADR verb
XADR	Verb		PMDF	Returns information about how an address is routed internally by PMDF as well as general channel information
XAUD	Keyword		NT Mail	I'm guessing this is an audit ID reporting licence number and version
XAUTH	Keyword	Microsoft bug report	Microsoft Exchange Server 5.0	Indicates support for the XAUTH verb
XAUTH	Verb	Microsoft bug report	Microsoft Exchange Server 5.0	Authentication. If accepted challenge/response pairs are sent 1 or more times.
XBDATA	Keyword		Mailbeamer	Details are unknown.
XCIR	Keyword		PMDF	Indicates support for the XCIR verb
XCIR	Verb		PMDF	Returns PMDF circuit check information
XDATASIZE	Keyword		LSMTP	I'm guessing it's an obsolete form of XLSMTP-DATASIZE
XDSN	Keyword	RFC1891	Zmailer,Sendmail	Indicates support for Delivery Status Notification by adding optional parameters to the MAIL and RCPT command
XEXCH50	Keyword	Microsoft	Microsoft Exchange Server	Indicates support for the XEXCH50 verb
XEXCH50	Verb	Microsoft	Microsoft Exchange Server	Allows transfer of binary data with Exchange specific recipient information (eg plain text only versus MIME, etc). If accepted, receiver SMTP servers sends 354 Send Binary data and sending SMTP server sends the number of bytes as the first parameter on the XEXCH50 command. Once these bytes are sent, the receiving SMTP server sends an acknowledgement
XGEN	Keyword		PMDF	Indicates support for the XGEN verb
XGEN	Verb		PMDF	Returns status information about whether a compiled configuration and compiled character set are in use
XLICENSE	Verb		LSMTP	Returns license information
XLSMTP-ACKRCPT	Keyword		LSMTP	I'm guessing it's linked with L-Soft's LISTSERV programme. There are probably associated keywords. Details are unknown.
XLSMTP-DATASIZE	Keyword		LSMTP	I'm guessing it's linked with L-Soft's LISTSERV programme. There are probably associated keywords. Details are unknown.
XLSMTP-MMERGE	Keyword		LSMTP	I'm guessing it's linked with L-Soft's LISTSERV programme and mail merge. There are probably associated keywords. Details are unknown.
XMBEAM30	Keyword		Mailbeamer	I'm guessing this simply identifies the software and version running. Details are unknown.
XONE	Keyword		Sendmail - Obsolete	Indicates support for the ONEX verb
XQUE	Keyword		Sendmail - Obsolete	Indicates support for the XQUE
XQUE	Verb		Sendmail - Obsolete	I'm guessing this is equivalent to ETRN
XREMOTEQUEUE	Keyword		Netscape Mail server	I'm guessing it indicates support for the XREMOTEQUEUE verb, to implement another version of the TURN command. Details are unknown.
XSBEAM30	Keyword		Mailbeamer	I'm guessing this simply identifies the software and version running. Details are unknown.
XSECURITY	Keyword	SECURITY SMTP Service Extension	Courier	Indicates support for the SECURITY parameter with the MAIL verb. This allows for specifying the secruity level for delivering mail.
XSTA	Keyword		PMDF	Indicates support for the XSTA verb
XSTA	Verb		PMDF	Returns status information about the number of messages processed and currently in the PMDF channel queues
XTRN	Keyword		SLMail	Indicates support for the XTRN verb
XTRN	Verb		SLMail	I believe this is equivalent to ETRN
XUSR	Keyword		Sendmail - Obsolete	Indicates support for the XUSR verb
XUSR	Verb		Sendmail - Obsolete	Indicates that the submitted mail is coming directly from the user, and not relayed from another SMTP server
XVERP	Keyword		Qmail,Postfix,Courier	Indicates support for the XVERP parameter with the MAIL verb. This allows for automatic inclusion of the intended recipients address in the MAIL FROM address, so that bounces clearly show the address of the intended recipient.
XVRB	Keyword		Sendmail	Indicates support for the VERB verb
XXWALL30	Keyword		XWall	I'm guessing this simply identifies the software and version running. Details are unknown.