Every email passes through a chain of specialized servers before reaching its destination.
The email journey: MUA → MSA → MTA → MTA → MDA → MUA
| Component | What it does | Port |
|---|---|---|
| MUA (Mail User Agent) | Your email client (Gmail, Outlook, Thunderbird) | — |
| MSA (Mail Submission Agent) | Accepts mail, requires authentication | 587 |
| MTA (Mail Transfer Agent) | Routes mail between servers via DNS MX lookups | 25 |
| MDA (Mail Delivery Agent) | Places message in recipient's mailbox | — |
Three ports every pro must know:
- Port 25 — Server-to-server relay (increasingly blocked for residential IPs)
- Port 587 — Standard submission port with STARTTLS
- Port 465 — Implicit TLS (encrypted from the first byte)
STARTTLS vs. Implicit TLS: STARTTLS is opportunistic — a man-in-the-middle can strip the TLS upgrade. Implicit TLS on port 465 starts encrypted and stays encrypted. This vulnerability is exactly what MTA-STS was designed to address.
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