MTA = Mail Transfer Agent. AKA "Email Server". MTAs: Sendmail, Postfix, MS Exchange, OpenSMTPD....
MUA = Mail User Agent. AKA "Email application". MUAs: Thunderbird, Mutt, MS Outlook, Eudora....
Quote:
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user********/var/mail/user***************Earthlink
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Yep. But when you mention
port 110, you are referring to the destination port used by
POP. POP and IMAP are the protocols used for MUA/MTA communication, solely for receipt of mail to the MUA.
[sending MTA] - {SMTP} - [Earthlink MTA] - {POP} - [Your MUA].
OpenSMTPD would not be involved in this at all.
For sending email, OpenSMTPD would be your local MTA for outbound traffic only. Outbound, MUAs use SMTP or ESMTP. For a Mutt client on the same platform, this might be no more than SMTP via localhost port 25.
[Your MUA] - {ESMTP/SMTP} - [OpenSMTPD] - {ESMTP/SMTP} - [Earthlink MTA] - {SMTP} - [Destination MTA]
In my case, Email comes to me through OpenSMTPD in this way:
[Sending MTA] - {ESMTP/SMTP} - [mailhop MTA] - {ESMTP} - [OpenSMTPD] -> local mboxes in /var/mail
Local MUAs use the mbox files for received mail, and communicate with OpenSMTPD directly only for outbound traffic.