Is SMTP simply a protocol or a specific MDA?

Question

https://web.archive.org/web/20150709231237/http://www.feep.net/sendmail/tutorial/intro/MUA-MTA-MDA.html

The diagram there shows SMTP MDA written together inside MTA sendmail box.
They have given mail.local, procmail as examples of MDA.

• Do they work according to SMTP protocol or SMTP is a specific kind of MDA different from those two?

• Does the diagram imply that a specific MDA named SMTP is already implemented in Sendmail?

Answer 1

SMTP stands for Simple Mail Transfer Protocol. So yes, it's a protocol. I assume "SMTP MDA" means "Mail Delivery Agent using SMTP", because in ancient times when sendmail was written there were also other protocols, which nobody uses today.

At least today, the sendmail package comes with all parts necessary, and that implies parts being able to use SMTP.

And whatever are you doing, please look at a more modern mailer than sendmail.

Answer 2

SMTP is a separate protocol designed for the transmitting and receiving of email [between mail servers]

An MDA is a process used to flter and process email and dump it to disk. This process is not part of SMTP but something a fully fledged mail server needs to be able to do to be able to handle incoming email.