Active MITM of the TLS connection is a problem and has happened on the scale of entire countries IIRC, so I hope you are doing "proper" verification, whatever t

Active MITM of the TLS connection is a problem and has happened on the scale of entire countries IIRC, so I hope you are doing "proper" verification, whatever that might mean for your two servers. TLS without active MITM protects the connection, but only between your email server and foobar.com. If someone@example.com is a forwarder to foo@bar.com and there is no TLS on that connection, then there is an issue. Same for unencrypted IMAP/POP3 connections from the laptop/phone endpoint to the final mail resting point. Of course, all the sysadmins on the chain have access to the data too. This is where OpenPGP or SMIME come in.