Secure SMTP/TLS SRV Announcement

Introduction defines a widely adopted DNS-based service discovery protocol. is a specification of SRV records for the email protocols IMAP, POP3, and SUBMISSION. SMTP connections to MTAs (, section 4.3.2) are yet excluded from their share on the widely supported SRV record. The SMTP/TLS SRV announcement can be queried before a connection to a discovered SMTP MX server is established; it MAY be used to further distribute SMTP load via the SRV Priority and Weight mechanism.

Requirements Language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here. The term "Implicit TLS" refers to the automatic negotiation of TLS whenever a TCP connection is made on a particular TCP port that is used exclusively by that server for TLS connections. The term "Implicit TLS" is intended to contrast with the use of the STARTTLS command in SMTP that is used by the client and the server to explicitly negotiate TLS on an established cleartext TCP connection.

SMTP/TLS SRV Service Name The service name for TLS enabled Secure SMTP is smtps, the resulting DNS label _smtps.

STARTTLS: Whenever a domain publishes an according DNS SRV resource record it asserts availability of Secure SMTP, that is, of the STARTTLS SMTP service extension on the normal SMTP port (specified by IANA as port 25). The port number MUST be given as 0.
Implicit TLS: If the port number of the published SRV resource record is not 0, then the domain announces to support Implicit TLS on the given port in addition to STARTTLS on the normal SMTP port. (Specifying the IANA SMTP port results in a non-compliant service, and SHOULD NOT be used on the internet.) The STARTTLS extension command MUST NOT be used during connections on an Implicit TLS port.

Example: service record Here are two examples, the first announcing to support the STARTTLS SMTP service extension, the second specifying the port number 26 to indicate an additional Implicit TLS port.

IANA Considerations IANA is not expected to add a dedicated port for "Implicit TLS SMTP". Shall it consider otherwise the author suggests port 26.

Security Considerations This specification can be used to avoid downgrade attacks on the opportunistic approach of STARTTLS, accomplished via the mechanism used for many other IETF standardized protocols, most notably (IMAP, POP3, SUBMISSION). With its Implicit TLS capability it grants the SMTP protocol the same level of confidentiality through TLS as is already standardized for the other email protocols; the fewer necessary packet roundtrips aid, too. The security of DNS is out of scope for this specification, but DNSSEC and secure DNS transport etc exists.