We are looking to migrate our existing Exchange mail system to a better one, but for many we've looked at we've hit a stumbling block.

Some of our customers require mandatory TLS when sending/receiving emails between our domains, but the only mention of TLS I've seen on the forums and in the technical guides for Zimbra is TLS between a client and the server, not between a server and a server.

Is it possible to implement mandatory TLS between mail servers using Zimbra?