This probably answers my own question but I'm still curious to hear if anyone else has an answer.

From the code I've read it seems that fetchmail.py in odoo/addons/fetchmail/ makes use of imaplib and poplib and both have missing support for STARTTLS. I don't know about a solution for poplib's missing starttls method but I found a fork of imaplib which is called imaplib2 that implements a starttls method. This could be used to add starttls functionality.

What is a possible solution if I would like to use STARTTLS with pop3?