SMTP components or Why sending emails is a rocket science
Originally published on 2022-11-10 at /posts/smtp-components/; republished here on 2026-09-23 with links updated.
Hey, Wanna know the biggest scam you can get in the email world (apart from “email is a decentralized communication protocol”)?
Simple Mail Transfer Protocol.
Simple, huh? They lied. It’s damn rocket science!
I’m just trying to compile a note about what I found while working on postmoogle (opens in new tab).
Let me just throw some smtp-related tags and see if you can correctly handle at least one of them: MUA, MSA, MTA, MDA.
Do you recall something? I bet you didn’t. That’s a description of the high-level flow on how email “Where is my order” sent from your personal@email.com to support@amazon.com (of course, all addresses are just examples. Not sure if they even exist).
Digging through Wikipedia (opens in new tab):
- MUA (opens in new tab) stands for
mail user agent - MSA (opens in new tab) stands for
mail submission agent - MTA (opens in new tab) stands for
mail transfer agent - MDA (opens in new tab) stands for
mail delivery agent
there are some other things, but they’re a bit out of the scope of how SMTP works
Still don’t get it? Ok, let me try to translate those M*As:
- MUA - email client (thunderbird, k-9 mail, apple mail)
- MSA - API endpoint where your email client sends data when you hit the “Send email” button. Input part of an “email server”
- MTA - a software to send emails received from users to other “email servers” (and their users). The output part of an “email server”
- MDA - reverse-MSA, a software that receives emails from other “email servers” and sends them to users (an “IMAP server input” thing)
So, how it works?
MUA (your mail client) sends a request to your email’s MSA with the email you sent, and MSA forwards/delivers it to your recipient’s MSA.
Wait, where are all those MTA, MDA, M*A things?
Well, they are part of the same “email server” program now. Nowadays an “email server” can be only one program that runs with different input params:
$ emailserver --be-msa
$ emailserver --be-mta
$ emailserver --be-mda
# ...
And to understand that, you have to dig through Wikipedia, try to implement every component, understand that you have a horrifying mess of things, dig through Wikipedia again, catch zen and implement everything as 1 program even without --be-X params.
Yeah, that’s how postmoogle (opens in new tab) was implemented - I didn’t have a clue about how email actually works.
Now I have :D