The only real benefit I see here is that your IP won't be easily revealed. That is, given a fastmail account, the e.g. FBI cannot quickly get your login IP, like they can with e.g. Outlook or Gmail. So, for just low-level anti-surveillance, SSL to fastmail might suffice instead of using Tor with Gmail.
Unless you're using PGP or S/MIME, SMTP is still most often unencrypted.
Unless you're using PGP or S/MIME, SMTP is still most often unencrypted.