I don't know what an MTA is

Mail Transport Agent. That piece of software that makes your mail travel from one computer to the other. Something like exim or sendmail. MTAs talk SMTP on TCP port 25.

The other abbreviations you frequently see in the context of mail handling are:

MUA, Mail User Agent

This is the software that lets you read and write mails. Thunderbird, in this case, or elm, pine, mail.

MSA, Mail Submission Agent

The software that accepts mails from the MUA and transfers it to the MTA. "Outgoing mail server". This function is often integrated with the MTA. Sendmail is (or once was) so dominant that many other MSAs and MTAs have a compatibility executable called sendmail that accepts sendmail most common options for submitting emails. MSAs talk SMTP on TCP port 587.

MDA, Mail Delivery Agent

The software that stuffs your mail into your mailbox. Typically procmail or maildrop, invoked by the final MTA.

MRA, Mail Retrieval Agent

Software that polls a remote mail server, fetches mails and either deliver the mails locally or re-submit it to a (local) mailserver. This is completely optional. Typical implementations are getmail and fetchmail.

Alexander