TL;DR: SMTP moves mail between servers (and from clients to servers). IMAP/POP move mail to the user for reading. Typical ports: 25 (server↔server), 587 (client submission with STARTTLS), 465 (implicit TLS, a.k.a. SMTPS), 2525 (provider fallback).

SMTP

Glossary

• SMTP — Simple Mail Transfer Protocol
• MTA — Mail Transfer Agent (relays mail to other servers)
• DNS — Domain Name System
• MSA — Mail Submission Agent (accepts mail from clients)
• MDA — Mail Delivery Agent (places mail into a mailbox)
• SSL — Secure Sockets Layer (legacy)
• TLS — Transport Layer Security (current standard)
• IMAP — Internet Message Access Protocol (mail retrieval)
• POP — Post Office Protocol (mail retrieval)

How does it work?

SMTP defines how email is handed off from a client or server to the next server.

1. A user’s email client connects to an MSA to submit a message.
2. The MTA inspects the recipient’s domain (e.g., user@example.com).
3. The MTA queries DNS for that domain’s MX record(s), then resolves those hosts to IP addresses.
4. The sending MTA opens an SMTP connection to the receiving server and transfers the message.
5. The receiving side hands the message to an MDA, which stores it in the recipient’s mailbox.

SMTP envelope vs. headers: The envelope (MAIL FROM/RCPT TO) tells servers where mail is from and where it’s going. It’s distinct from the message headers/body that recipients see (e.g., From:, To:, Subject:).

Minimal flow (simplified)

client → MSA → MTA (sender) → MTA (recipient) → MDA → mailbox → IMAP/POP → user

SMTP servers & roles

• MSA: Receives mail from clients, enforces auth/rate limits/policies.
• MTA: Relays mail to the next hop based on DNS (MX) lookup and policy.
• MDA: Accepts final delivery and writes to the user’s mailbox store.

Ports SMTP uses

• 25 — Primary server-to-server transport. Often blocked on residential/enterprise egress to prevent abuse.
• 587 — Message submission for clients. Use STARTTLS and authentication.
• 465 — Implicit TLS (SMTPS). Common in the wild; some providers require/offer it.
• 2525 — Non-standard, but many providers support it as an alternative when 25/587 are blocked.

Note: SSL is deprecated; use TLS. Prefer authenticated submission on 587 with STARTTLS when available.

SMTP vs. IMAP/POP

SMTP moves messages between servers (and from client → server). To read mail, clients connect to the mailbox using IMAP (synchronizes folders, supports multiple devices) or POP (downloads, typically simpler/one-device workflows).