Port 25 is the TCP port assigned to SMTP (Simple Mail Transfer Protocol) since RFC 821 in 1982. It’s still the port used for server-to-server mail relay across the internet — the port your MX records point to receive mail.
What Runs on Port 25
- Receiving mail — every internet-facing MX server listens on port 25 to receive mail from other domains
- Server-to-server relay — MTAs communicate with each other over port 25
- Historically, mail submission — modern clients no longer use it (see below)
Why ISPs Block Outbound Port 25
Most residential and cellular ISPs — Comcast, AT&T, Verizon, T-Mobile, cloud providers like AWS EC2 — block outbound port 25 by default. Reason: any compromised home PC or VPS could otherwise spew spam directly to remote MX servers. Blocking port 25 forces spammers to route through an authenticated ISP relay or provider SMTP, where abuse can be caught.
Modern Mail Submission
Clients should use:
- Port 587 — Submission with STARTTLS (RFC 6409, current standard)
- Port 465 — SMTPS (implicit TLS, revived after decades of deprecation)
Not port 25.
When Port 25 Matters to You
- Setting up an inbound MX — port 25 must be open on your MTA to receive mail
- Debugging why a self-hosted mail server can’t send outbound — your ISP probably blocks port 25 outbound
Check whether port 25 is open on any host with the port scanner.
Related
Check the Port 465 glossary entry, the Port 587 entry, and read what an SPF record is — SPF authenticates mail flowing over port 25.