SMTP Ports 25, 587 & 465 Explained
Port 25, 587 or 465 — picking the wrong SMTP port breaks sending or triggers security blocks. Here's what each port is for.
TL;DR
Port 25 = server-to-server delivery (MTA to MTA). Port 587 = authenticated submission (the modern default for apps). Port 465 = implicit-TLS submission. For your sending server, outbound delivery uses 25.
The three ports at a glance
| Port | Purpose | Encryption |
|---|---|---|
| 25 | MTA-to-MTA delivery | Opportunistic STARTTLS |
| 587 | Client → server submission | STARTTLS |
| 465 | Client → server submission | Implicit TLS |
Port 25 — delivery between servers
This is how your MTA hands mail to Gmail, Outlook and Yahoo. Your PowerMTA server needs outbound port 25 open to deliver. Many hosts block it by default — see how to fix port 25 blocked.
Port 587 — modern submission
When an app or email client submits a message to your server, it should use 587 with authentication and STARTTLS. This is the standard "submission" port.
Port 465 — implicit TLS submission
Once deprecated, 465 was reinstated for implicit-TLS submission. It's functionally similar to 587 but encrypts from the first byte. Either is fine for submission; pick what your client supports.
Which port for bulk sending?
Your outbound bulk delivery rides on port 25 server-to-server. Apps feeding your MTA use 587/465. Confirming 25 is open is step one of any delivery setup.
AutoPMTA configures and tests your mail ports during install so sending works the first time. View plans.
Key takeaways
- Port 25 = server-to-server delivery (outbound for your MTA).
- Port 587 = authenticated client submission (modern default).
- Port 465 = implicit-TLS submission alternative.
- Bulk delivery needs outbound 25 open.
Frequently asked questions
Which SMTP port should I use for sending email?+
Server-to-server delivery uses port 25. Applications submitting mail to a server should use port 587 (STARTTLS) or 465 (implicit TLS) with authentication.
Why is port 25 blocked?+
Many hosting providers block outbound port 25 by default to prevent spam from compromised servers. You usually need to request that it be unblocked for legitimate email sending.
Is port 587 or 465 better?+
Both are secure submission ports. Port 587 uses STARTTLS and is the modern standard; 465 uses implicit TLS. Use whichever your client or application supports.