Squirrelmail was actually using SSL for quite a while... it just wasn't official.
TLS SMTP isn't that difficult to setup (it's already possible on our system, as the version of Postfix we're using has the TLS patches)....
As with everything else, it's dealing with the certificate warnings (and having yet another thing to break / have to support etc.) that's the real problem... Microsoft products are especially annoying to deal with in this regard, and of course that's what 99% of people use.
I would like to see us offering this within the next few months though, and it shouldn't be that difficult to do.