Simple Mail Transfer Protocol (SMTP)

Simple Mail Transfer Protocol (SMTP)
Simple Mail Transfer Protocol (SMTP)
2025-09-19
Note
This registry group was formerly known as "MAIL Parameters."
Associated Registries
Available Formats
XML
HTML
Plain text
Registries Included Below
SMTP Service Extensions
SMTP Service Extension Parameters
Mail Transmission Types for the "Received:" Header Field
VIA Link Types
WITH Protocol Types
Additional Registered 'Received:' Clauses
Address Literal Tags
SMTP PRIORITY Extension Priority Assignment Policy
Registered-states
Multicast Email SMTP Extensions
SMTP Server Limits
Mail Header Confidentiality Policies
SMTP Service Extensions
Registration Procedure(s)
Either Model 1 (IETF) or Model 2 (FCFS) as described in Sections 8.1.1.1 of [
RFC-ietf-emailcore-rfc5321bis-43
].
Reference
RFC-ietf-emailcore-rfc5321bis-43, Sections 8.1.1, 2.2.2
Note
The Simple Mail Transfer Protocol [
RFC-ietf-emailcore-rfc5321bis-43
specifies a set of commands or services and a general procedure for
extending that set. The table below lists SMTP service extensions.
Both message submission [
RFC6409
]and mail transfer
RFC-ietf-emailcore-rfc5321bis-43
] may use these extensions unless
otherwise specified.
Note
The SMTP Extension names/keywords registered in this section are for
general use. For those specific to Multicast email, see the registry
for that protocol below.
Available Formats
CSV
EHLO Keyword
Description
Reference
EHLO Parameters
Additional Verbs
MAIL/RCPT Parameter Values
RegMethod
Message Submission Use and Values
Length Added
Note
Contact
Change Controller
SEND
Send to terminal
RFC821
None
RFC821
RFC821
IETF
MUST NOT
The description of SEND was updated by
RFC1123
] and then its actual use was
deprecated in [
RFC2821
IETF
IETF
SOML
Send as mail or to terminal
RFC821
None
RFC821
RFC821
IETF
MUST NOT
The description of SOML was updated by
RFC1123
] and then its actual use was
deprecated in [
RFC2821
IETF
IETF
SAML
Send as mail and to terminal
RFC821
None
RFC821
RFC821
IETF
MUST NOT
The description of SAML was updated by
RFC1123
] and then its actual use was
deprecated in [
RFC2821
IETF
IETF
VRFY
VRFY command as specified in [
RFC-ietf-emailcore-rfc5321bis-43
RFC-ietf-emailcore-rfc5321bis-43
None
None
None
IETF
MUST NOT
Implementation support for VRFY is required in servers, but its listing in the EHLO response is optional. See Section 3.5.2 in [
RFC-ietf-emailcore-rfc5321bis-43
] for details on this subject.
IETF
IETF
EXPN
Expand the mailing list
RFC821
None
RFC821
RFC821
IETF
MUST NOT
Command description updated by [
RFC-ietf-emailcore-rfc5321bis-43
IETF
IETF
HELP
Supply helpful information
RFC821
None
RFC821
RFC821
IETF
MUST NOT
Command description updated by [
RFC-ietf-emailcore-rfc5321bis-43
IETF
IETF
TURN
Interchange the client and server
roles
RFC821
None
RFC821
RFC821
IETF
MUST NOT
The actual use of TURN was deprecated in
RFC2821
IETF
IETF
8BITMIME
SMTP and Submit transport of 8bit
MIME content
RFC6152
None
RFC6152
RFC6152
IETF
SHOULD
IETF
IETF
SIZE
Message size declaration
RFC1870
number
RFC1870
RFC1870
IETF
MAY
For additional declarations or limits of this general type,
see the LIMITS extension below and the SMTP Server Limits
registry.
IETF
IETF
VERB
Verbose
[Eric Allman]
Not supplied
Not supplied
Not supplied
Legacy
Not supplied
Not supplied
Eric Allman
Eric Allman
ONEX
One message transaction only
[Eric Allman]
Not supplied
Not supplied
Not supplied
Legacy
Not supplied
Not supplied
Eric Allman
Eric Allman
CHUNKING
Chunking
RFC3030
None
RFC3030
RFC3030
IETF
MAY
IETF
IETF
BINARYMIME
Binary MIME
RFC3030
None
RFC3030
RFC3030
IETF
MAY
IETF
IETF
CHECKPOINT
Checkpoint/Restart
RFC1845
None
RFC1845
RFC1845
IETF
MAY
IETF
IETF
DELIVERBY
Deliver By
RFC2852
None
RFC2852
RFC2852
IETF
MAY
IETF
IETF
PIPELINING
Command Pipelining
RFC2920
None
RFC2920
RFC2920
IETF
SHOULD
IETF
IETF
DSN
Delivery Status Notification
RFC3461
None
RFC3461
RFC3461
IETF
SHOULD
IETF
IETF
ETRN
Extended Turn
RFC1985
None
RFC1985
RFC1985
IETF
MUST NOT
SMTP [
RFC-ietf-emailcore-rfc5321bis-43
] only. Not for use on Submit port 587.
IETF
IETF
ENHANCEDSTATUSCODES
Enhanced Status Codes
RFC2034
None
RFC2034
RFC2034
IETF
SHOULD
IETF
IETF
STARTTLS
Start TLS
RFC3207
None
RFC3207
RFC3207
IETF
MAY
IETF
IETF
NO-SOLICITING
Notification of no soliciting
RFC3865
Soliciting-keywords
RFC3865
RFC3865
IETF
MAY
IETF
IETF
MTRK
Message Tracking
RFC3885
RFC3885
RFC3885
IETF
MAY
IETF
IETF
SUBMITTER
SMTP Responsible Submitter
RFC4405
None
RFC4405
RFC4405
IETF
MUST NOT
Deprecated by [
Moving RFC 4405, RFC 4406, RFC 4407 (Sender-ID) to Historic
].
IETF
IETF
ATRN
Authenticated TURN
RFC2645
None
RFC2645
RFC2645
IETF
MUST NOT
SMTP [
RFC-ietf-emailcore-rfc5321bis-43
] only. Not for use on Submit port 587.
IETF
IETF
AUTH
Authentication
RFC4954
SASL mechanism names
RFC4954
RFC4954
IETF
MUST
IETF
IETF
BURL
Remote Content
RFC4468
Permitted URL prefixes
RFC4468
RFC4468
IETF
MUST NOT
Submit [
RFC6409
] only. Not for use with SMTP on port 25.
IETF
IETF
FUTURERELEASE
Future Message Release
RFC4865
None
RFC4865
RFC4865
IETF
MUST NOT
IETF
IETF
UTF8SMTP
Internationalized email address
RFC5336
None
RFC5336
RFC5336
IETF
MUST NOT
Experimental; deprecated in [
RFC6531
].
IETF
IETF
CONPERM
Content Conversion Permission
RFC4141
None
RFC4141
RFC4141
IETF
MAY
IETF
IETF
CONNEG
Content Conversion Negotiation
RFC4141
None
RFC4141
RFC4141
IETF
MAY
IETF
IETF
SMTPUTF8
Internationalized email address
RFC6531
None
RFC6531
RFC6531
IETF
MUST NOT
IETF
IETF
MT-PRIORITY
Priority Message Handling
RFC6710
Name of the Priority Assignment Policy (optional)
RFC6710
RFC6710
IETF
MUST NOT
IETF
IETF
RRVS
Require Recipient Valid Since
RFC7293
None
RFC7293
RFC7293
IETF
MUST NOT
IETF
IETF
REQUIRETLS
Require TLS
RFC8689
None
RFC8689
RFC8689
IETF
MUST NOT
IETF
IETF
LIMITS
Server limits
RFC9422
None
RFC9422
RFC9422
IETF
MUST NOT
See "SMTP Server Limits" registry.
IETF
IETF
SMTP Service Extension Parameters
Registration Procedure(s)
Registry closed
Reference
RFC-ietf-emailcore-rfc5321bis-43
Note
These entries have been incorporated into the SMTP Service Extension registry's
"EHLO Parameters" field.
Mail Transmission Types for the "Received:" Header Field
Reference
RFC-ietf-emailcore-rfc5321bis-43, Sections 8.1.3, 8.1.4
][
RFC-ietf-emailcore-rfc5322bis-12
Note
The Simple Mail Transfer Protocol [
RFC-ietf-emailcore-rfc5321bis-43
and the Standard for Internet Message Formats [
RFC-ietf-emailcore-rfc5322bis-12
specify that a set of "Received" lines will be prepended to the
headers of electronic mail messages as they are transported through
the Internet. These received line may optionally include either or
both a "via" phrase and/or a "with" phrase. The legal values for the
phrases are listed here. The via phrase is intended to indicate the
link or physical medium over which the message was transferred. The
with phrase is intended to indicate the protocol or logical process
that was used to transfer the message.
VIA Link Types
Registration Procedure(s)
RFC Required
Reference
RFC-ietf-emailcore-rfc5321bis-43
Available Formats
CSV
VIA link types
Description
Reference
UUCP
Unix-to-Unix Copy Program
TCP
TCP
RFC-ietf-emailcore-rfc5321bis-43
WITH Protocol Types
Registration Procedure(s)
RFC Required
Reference
RFC-ietf-emailcore-rfc5321bis-43
Available Formats
CSV
WITH protocol types
Description
Reference
SMTP
Simple Mail Transfer Protocol
RFC-ietf-emailcore-rfc5321bis-43
ESMTP
SMTP with Service Extensions
RFC-ietf-emailcore-rfc5321bis-43
ESMTPA
ESMTP with AUTH
RFC3848
ESMTPS
ESMTP with STARTTLS
RFC3848
ESMTPSA
ESMTP with both STARTTLS and AUTH
RFC3848
LMTP
Local Mail Transfer Protocol
RFC3848
LMTPA
LMTP with AUTH
RFC3848
LMTPS
LMTP with STARTTLS
RFC3848
LMTPSA
LMTP with both STARTTLS and AUTH
RFC3848
MMS
Multimedia Messaging Service
RFC4356
UTF8SMTP
ESMTP with SMTPUTF8
RFC6531
UTF8SMTPA
ESMTP with SMTPUTF8 and AUTH
RFC4954
][
RFC6531
UTF8SMTPS
ESMTP with SMTPUTF8 and STARTTLS
RFC3207
][
RFC6531
UTF8SMTPSA
ESMTP with SMTPUTF8 and both STARTTLS and
AUTH
RFC3207
][
RFC4954
][
RFC6531
UTF8LMTP
LMTP with SMTPUTF8
RFC6531
UTF8LMTPA
LMTP with SMTPUTF8 and AUTH
RFC4954
][
RFC6531
UTF8LMTPS
LMTP with SMTPUTF8 and STARTTLS
RFC3207
][
RFC6531
UTF8LMTPSA
LMTP with SMTPUTF8 and both STARTTLS and
AUTH
RFC3207
][
RFC4954
][
RFC6531
Additional Registered 'Received:' Clauses
Registration Procedure(s)
IETF Review
Reference
RFC-ietf-emailcore-rfc5321bis-43
Available Formats
CSV
Clause Name
Service Extension
Description
Syntax Summary
Reference
PRIORITY
Records the value of the MT-PRIORITY
parameter specified in the MAIL FROM
command
See Section 7 of RFC [
RFC6710
RFC6710
state
Indicates entry into a special
queue state
state
RFC6729
tls
Indicates the TLS cipher used
tls-cipher = tls-cipher-name / tls-cipher-hex
tls-cipher-name = ALPHA *(ALPHA / DIGIT / "_")
; as registered in the IANA TLS Cipher Suite Registry
tls-cipher-hex = "0x" 4HEXDIG
RFC8314
group
Indicates the Diffie-Hellman group used with the TLS cipher (if applicable)
dh-group = ALPHA *(ALPHA / DIGIT / "_" / "-")
; as registered in the IANA TLS Supported Groups Registry
RFC8314
Address Literal Tags
Registration Procedure(s)
Standards Action
Reference
RFC-ietf-emailcore-rfc5321bis-43
Note
Sometimes a host is not known to the domain name system and
communication (and, in particular, communication to report and repair
the error) is blocked. To bypass this barrier a special literal form
of the address is allowed as an alternative to a domain name. For
IPv4 addresses, this form uses four small decimal integers separated
by dots and enclosed by brackets such as [123.255.37.2], which
indicates an (IPv4) Internet Address in sequence-of-octets form. For
IPv6 and other forms of addressing that might eventually be
standardized, the form consists of a standardized "tag" that
identifies the address syntax, a colon, and the address itself, in a
format specified as part of the IPv6 standards [
RFC5952
].
Available Formats
CSV
Tag
Reference
IPv6
RFC-ietf-emailcore-rfc5321bis-43
SMTP PRIORITY Extension Priority Assignment Policy
Registration Procedure(s)
Specification Required
Expert(s)
Unassigned
Reference
RFC6710
Available Formats
CSV
Policy Name
Comment
Reference
MIXER
Default policy
RFC6710, Appendix B
STANAG4406
RFC6710, Appendix A
NSEP
RFC6710, Appendix C
Registered-states
Registration Procedure(s)
First Come First Served
Reference
RFC6729
Available Formats
CSV
Name
Description
Reference
Use
auth
Held for message authentication
RFC6729
current
content
Held for message content analysis
RFC6729
current
convert
Held for message content conversion
RFC6729
current
moderation
Held for list moderation
RFC6729
current
normal
Message is not being held other than to
accommodate typical relaying handling
RFC6729
current
other
Held for causes not covered by other
registered state keywords
RFC6729
current
outbound
Message placed in outbound queue
RFC6729
current
quarantine
Held for operator action due to content
analysis or local policy
RFC6729
current
timed
Held to accommodate a specific requested
delivery window
RFC6729
current
Multicast Email SMTP Extensions
Registration Procedure(s)
Specification Required
Expert(s)
Alexey Melnikov (primary), RFC Independent Submissions Editor (secondary)
Reference
RFC8494
Note
Extension names/keywords and status used with Multicast Email; see
SMTP Service Extensions above for the list for more general use.
Available Formats
CSV
Name
Status
Multicast-Mail-Specific References
Non-Multicast-Specific References
Note
SIZE
Required
RFC1870
8BITMIME
Required
RFC6152
DSN
Required
RFC3461
MT-PRIORITY
Required
RFC6710
DELIVERBY
Required
RFC2852
BINARYMIME
Required
RFC3030
CHUNKING
Special
RFC3030
SMTP CHUNKING MUST be supported on the receiving SMTP side of
an SMTP-to-MULE gateway and MAY be used on the sending side of a
MULE-to-SMTP gateway. A MULE relay doesn't need to do anything
special for this extension.
ENHANCEDSTATUSCODES
Special
RFC2034
The ENHANCEDSTATUSCODES extension is supported by including
relevant status codes in DSN [
RFC3461
] reports.
RRVS
Supported
RFC7293
SUBMITTER
Supported
RFC4405
Deprecated by [
Moving RFC 4405, RFC 4406, RFC 4407 (Sender-ID) to Historic
].
PIPELINING
N/A
RFC2920
STARTTLS
N/A
RFC3207
AUTH
Special
RFC4954
The AUTH parameter to the MAIL FROM command is "Supported",
but the rest of the AUTH extension is not applicable to MULE.
BURL
N/A
RFC4468
NO-SOLICITING
N/A
RFC3865
CHECKPOINT
Disallowed
RFC1845
CONNEG
Disallowed
RFC4141
SMTP Server Limits
Registration Procedure(s)
Specification Required
Expert(s)
John Klensin, Murray Kucherawy (backup)
Reference
RFC9422
Available Formats
CSV
Name
Value syntax
Description
Restrictions
Security Considerations
Reference
MAILMAX
%x31-39 0*5DIGIT ; "0" not allowed, six-digit maximum
MAILMAX specifies the maximum number of transactions
(MAIL FROM commands) the server will accept in a single session.
The count includes all MAIL FROM commands, regardless of whether
they succeed or fail.
None
See [
RFC9422, Section 6
RFC9422
RCPTMAX
%x31-39 0*5DIGIT ; "0" not allowed, six-digit maximum
RCPTMAX specifies the maximum number of RCPT TO commands
the server will accept in a single transaction. It is not a limit on
the actual number of recipients the message ends up being sent to; a
single RCPT TO command may produce multiple recipients or, in the
event of an error, none.
None
See [
RFC9422, Section 6
RFC9422
RCPTDOMAINMAX
%x31-39 0*5DIGIT ; "0" not allowed, six-digit maximum
RCPTDOMAINMAX specifies the maximum number of different
domains that can appear in a recipient (RCPT TO) address within a
single session. This limit is imposed by some servers that bind to
a specific internal delivery mechanism on receipt of the first RCPT
TO command.
None
See [
RFC9422, Section 6
RFC9422
Mail Header Confidentiality Policies
Registration Procedure(s)
Specification Required or IETF Review (see Note)
Expert(s)
Daniel Kahn Gillmor (primary), Bernie Hoeneisen (secondary)
Reference
RFC9788
Note
Adding an entry to this registry with an N in the "Recommended"
column follows the registration policy of Specification Required.
Adding an entry to this registry with a Y in the "Recommended"
column or changing the "Recommended" column in an existing entry
(from N to Y or vice versa) requires IETF Review.
Note
The Header Confidentiality Policy Name never appears on the wire.
This registry merely tracks stable references to implementable
descriptions of distinct policies. Any addition to this registry
should be governed by guidance in Section 3.4.2 of
RFC9788
].
Available Formats
CSV
Header Confidentiality Policy Name
Description
Recommended
Reference
hcp_no_confidentiality
No header confidentiality
RFC9788, Section 3.2.3
hcp_baseline
Confidentiality for Informational Header Fields: Subject Header Field is obscured, Keywords and Comments are removed
RFC9788, Section 3.2.1
hcp_shy
Obscure Subject, remove Keywords and Comments, remove the time zone from Date, and remove display-names from From, To, and Cc
RFC9788, Section 3.2.2