Encryption for e-mail local delivery

Hello!

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?
papa kotaAsked:
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ArneLoviusCommented:
The delivery of mail over the internet can happen within STARTTLS

the MTA (Postfix/Sendmail/QMAIL) does not encrypt mail in this scenario, the mail is sent in plain text, but the communication between the mail servers that carries the email is encrypted.

After delivery from the remote MTA, the email exists on the server in a plain text form. The email is not "sent" to Courier-IMAP, or the MUA such as Thunderbird, but rather Thunderbird connects to courier (hopefully over IMAPS) which then opens the "mailbox" effectively "proxying" access to the mailbox, again the connection is in plaintext, with the communication encrypted with IMAPS.

IMAP/IMAPS and SMTP/SMTP STARTTLS have a similar relationship to HTTP/HTTPS

If Postfix, Courier-IMAP and Thunderbird are all of the same single user machine connecting over localhost, then the IMAPS certificate is less of an issue as sniffing the traffic across a network is no longer possible, but it still should be used in case somebody is able to run a a packet sniffer on the computer.

If you have a public certificate for SMTP STARTTLS, then I would usually use the same certificate for IMAPS
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