This all might be a bit confusing to understand at first, but it’s actually quite straight forward once you get to know all of the terms. Included some more information below which will help you to remember some of the terms and their role in a mail server.
MTA – Mail Transfer Agent – Sendmail, Postfix
MDA – Mail Delivery Agent – Procmail
MUA – Mail User Agent – Outlook, Thunderbird
SMTP – Simple Mail Transfer Protocol – Sendmail, Postfix
POP3 – Post Office Protocol 3 – Dovecot
IMAP – Internet Message Access Protocol – Dovecot