I run my own Postfix server. Recently I tried to enable SSL and TLS authorization so my Palm Pre phone can login with name and password and send outgoing mail from any network. There was no trouble setting up Thunderbird to use SSL authentication for SMTP, but the Palm Pre was very frustrating to figure out. It turns out that my GoDaddy SSL Certificate was not automatically trusted by the Palm Pre mail client. You have to import the certificate (self-signed or "real" certs) for it to successfully use encrypted SMTP. The following steps worked for me.
Make your postfix server allow relaying with SSL SASL authentication with names and passwords. (Look at master.cf to enable optional port 465 and 587 too.)
Test it with Thunderbird to be sure it is working, in my case port 465 with SSL.
Send an e-mail to yourself, attach the SSL certificate (public key) used by your postfix server.
Read that mail on your Palm Pre, download and open the attached cert file. Import into the Certificate Manager.
Email > Preferences & Accounts > [Your Account] > Change Login Settings (at bottom) > Outgoing Mail Server