Email servers explained: Postfix, Exim and the rest

An email server (mail server) is the software that accepts, routes, stores and hands out email. When you send a message, it does not go straight to the other person — it passes through one or more mail servers that find the recipient, decide whether to accept the mail, and place it in a mailbox. Understanding the pieces makes it clear why an address can look perfectly valid yet still bounce.

1. The three roles inside “a mail server”

• MTA — Mail Transfer Agent: sends and relays mail between servers over SMTP. This is the part our checker talks to. Examples: Postfix, Exim, Sendmail.
• MDA — Mail Delivery Agent: takes accepted mail and drops it into the right mailbox. Often built into or paired with the MTA (e.g. Dovecot's LMTP).
• Mailbox / access server: lets the user read mail with IMAP or POP3 (and powers webmail). Example: Dovecot.

A “mail server” people talk about is usually a stack of these working together, plus DNS records (MX, SPF, DKIM, DMARC) that tell the world where and how to send.

2. The journey of one email

Your app  ──SMTP──▶  Sending MTA  ──SMTP──▶  Recipient MX (MTA)
                                                   │
                                          spam & policy checks
                                                   ▼
                                          MDA ──▶ Mailbox store
                                                   ▲
                              User reads via IMAP / POP3 / webmail

The MX record points to the recipient's MTA. That MTA decides 250 (accept), 550 (no such user) or 4xx (try later). That single decision is the heart of email verification.

3. MTA software (the SMTP side)

4. Mailbox access & all-in-one stacks

IMAP/POP3 servers let people actually open their mail: Dovecot (by far the most common), Courier and Cyrus. All-in-one stacks bundle MTA + IMAP + webmail + anti-spam so you don't assemble them by hand:

• Mailcow and iRedMail — popular self-hosted stacks (Postfix + Dovecot + Rspamd + webmail).
• Mail-in-a-Box — one-command personal mail server.
• Zimbra and SOGo — groupware (mail + calendar + contacts) with webmail.

5. Hosted email (you don't run the server)

Most domains today don't run their own MTA at all — they point their MX records at a provider:

• Google Workspace (Gmail) — MX at aspmx.l.google.com.
• Microsoft 365 (Exchange Online) — MX at <domain>.mail.protection.outlook.com.
• Zoho, Proton, Fastmail, Yandex, iCloud+ — each with recognisable MX hostnames.
• Amazon SES, SendGrid, Mailgun, Postmark — sending services for transactional/bulk mail (often a different system from where the domain receives).

6. Spam & security gateways

Larger domains put a filtering layer in front of the real mailbox: Proofpoint (pphosted.com), Mimecast, Barracuda, Cisco/IronPort, or software filters like Rspamd and SpamAssassin. Their server is what the MX record points to, so they're the first thing a checker meets — and they often answer cautiously to discourage probing.

7. How to tell what a domain uses

Two reliable clues, both public:

• MX hostname pattern — the names returned by an MX lookup often give the provider away instantly (Google, Microsoft, Proofpoint, Zoho…).
• SMTP banner — on connect, an MTA greets with a 220 line that frequently names the software, e.g. 220 mail.example.com ESMTP Postfix or ... ESMTP Exim 4.96 or Microsoft ESMTP MAIL Service.

This is exactly the layer our tools inspect. Run a domain through the MX Lookup to see its mail servers, or check a specific address with the email checker. See also types of web servers — the website and the mail server are separate systems.