12.4.8 Transport Layer Security

Keywords: maytls, maytlsclient, maytlsserver, musttls, musttlsclient, musttlsserver, notls, notlsclient, notlsserver, tlsswitchchannel

The maytls, maytlsclient, maytlsserver, musttls, musttlsclient, musttlsserver, notls, notlsclient, notlsserver, and tlsswitchchannel channel keywords are used to configure TLS use during the SMTP protocol by SMTP based channels such as TCP/IP channels.

The default is notls, and means that TLS will not be permitted or attempted. It subsumes the notlsclient keyword, which means that TLS use will not be attempted by the MTA SMTP client on outgoing connections (the STARTTLS command will not be issued during outgoing connections) and the notlsserver keyword, which means that TLS use will not be permitted by the MTA SMTP server on incoming connections (the STARTTLS extension will not be advertised by the SMTP server nor the command itself accepted).

Specifying maytls causes the MTA to offer TLS to incoming connections and to attempt TLS upon outgoing connections. It subsumes maytlsclient, which means that the MTA SMTP client will attempt TLS use when sending outgoing messages, if sending to an SMTP server that supports TLS, and maytlsserver, which means that the MTA SMTP server will advertise support for the STARTTLS extension and will allow TLS use when receiving messages.

Note that for TLS to work, the following conditions must be in place:

• Protections/ownerships of the certificates have to be set so that the mailsrv account can access the files.

• The directory where the certificates are stored need to have protections/ownerships set such that the mailsrv account can access the files within that directory.

Specifying musttls will cause the MTA to insist upon TLS in both outgoing and incoming connections; email will not be exchanged with remote systems that fail to successfully negotiate TLS use. It subsumes musttlsclient, which means that the MTA SMTP client will insist on TLS use when sending outgoing messages and will not send to SMTP servers that do not successfully negotiate TLS use (the MTA will issue the STARTTLS command and that command must succeed). It also subsumes musttlsserver, which means that the MTA SMTP server will advertise support for the STARTTLS extension and will insist upon TLS use when receiving incoming messages and will not accept messages from clients that do not successfully negotiate TLS use.

The tlsswitchchannel keyword is used to cause incoming connections to be switched to a specified channel upon a client's successful TLS negotiation. It takes a required value, specifying the channel to which to switch.