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: 
SMTP stands for [Simple Mail Transfer Protocol](https://en.wikipedia.org/wiki/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`.
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.