In my understanding, most of email servers use SMTP/POP/IMAP over SSL to encrypt the email.
It supports encryption when client (UA) send email to server (MTA) and UA receive email from MTA. However, not so many MTAs can encrypt when they send email between MTA to MTA.
(is my understanding correct?)
firstname.lastname@example.org send email to email@example.com
[Alice's PC] --- encrypted (SMTPS) ---> [somewhere.com server] --- NOT ENCRYPTED (SMTP) ---> [anywhere.org server] --- encrypted (POPS or IMAPS) ---> [Bob's PC]
If my understanding is correct, why most email servers don't support SMTP over SSL between email servers?
I develop better (less complex) interface to enable email encryption with PGP/GPG, but these days I think it may be better to use SMTPS because PGP/GPG needs manual key signing to keep reliability.