I think that GnuPG copes with any valid public keyserver.
Back to the original question (IMAP and Avast):
Did some testing: Outlook Express-Avast-Stunnel-ISP with SSL IMAP,
and yes it does work.
Avast deleted Swen from a message!
So no need to port SSL support into Avast, when You can use free /GPL software that is already here.