Because my domain is HSTS, external SSL support needed to be set up manually for Postfix (the SMTP server) and Dovecot (the IMAP/POP3 server) as well as the web interface.
Also, I'm using SendGrid's free tier as my sending proxy from Postfix, because I don't want to deal with the headache of domain/IP reputation.
I use Modoboa as the basis for my mail server. It's a rolled-up system built on Postfix and Dovecot. In an effort to avoid the deliverability issues traditionally associated with self-hosted e-mail, I'm relaying SMTP through Send Grid.
Here are some config snippets that were needed to get this up and running.