It's opportunist encryption, which is worse than worthless, as it gives a false sense of security. All you need to defeat this encryption is to interfere in any way with the encrypted connection, SMTP is required to deliver the mail in plain text.
GPG is not a real solution as even no one among technically minded people I know uses it for encryption. Signatures, yes, especially in Debian where around 50% of posts on mailining lists are signed, but, I recall exactly one case when a piece of sensitive data I received was GPG encrypted.
But. an easy solution does exist: DANE. It's the only way to make that opportunist encryption mandatory (servers are required to abort delivery in face of failure), and DNSSEC prevents DANE settings from being stripped away by an attacker. Obviously, you need stapled certificates rather than mere CA selection, but that's common sense. With that, server->server and possibly client->server communication is secure, and when IMAP is protected by DANE, server->client as well. Local storage remains in plain text which is an obvious problem, but at least that is outside the topic of this discussion.
The problem is, I'm not aware of any mail software that actually uses DANE yet