The SMTP protocol is used both from client to server and from server to server. RFC 3207 (2002) complaint MTAs communicate with each other over a TLS (i.e. encrypted) connection.
Complaint MTAs include: sendmail (>= 8.11), postfix (>= 2.2), MS Exchange (>= 5.5). Patches have existed for qmail to add support since 1.01, though they aren't in the main distribution for reasons that I'm sure make sense to djb.
Complaint MTAs include: sendmail (>= 8.11), postfix (>= 2.2), MS Exchange (>= 5.5). Patches have existed for qmail to add support since 1.01, though they aren't in the main distribution for reasons that I'm sure make sense to djb.