I understand well what happens when SMTP server, say Postfix contacts remote SMTP server to send or receive mail. The connection could (and should) be encrypted, certain ports must be used and the certificate must be valid and accepted by all parties.
But when it comes to the local e-mail delivery for me it's a grey area. Ok, let's say if we're talking about receiving mail and Postfix already got it from the remote server. Then we have MDA (like Courier) that takes the mail from Postfix and gives it to a MUA (like Thunderbird Mail, for example). I'm talking about one physical machine and one user session. What the purpose of the encryption if it all happens inside and no third party could be present? And if it's the case of the same computer and the same user, then does the validity of the cert play any role for e-mail delivery process from Postfix via Courier to Thunderbird Mail?