SMTP ports are network endpoints used for email submission or relay.
If you are choosing between , , , and , start with this practical rule:
+ STARTTLS for most app sendersfor implicit TLS when provider requires itas fallback where supportedmainly for server-to-server relay
SMTP ports quick table
| Port | Typical role | Encryption model | App submission default |
|---|---|---|---|
| Relay between servers | Optional/varies | No |
| Submission | Implicit TLS | Sometimes |
| Submission | STARTTLS | Yes |
| Alternate submission | Provider-specific | Fallback |
Port-by-port guidance
Port 587
Best first choice for authenticated application submission.
Port 465
Use when provider expects TLS from first packet (implicit TLS).
Port 25
Use for relay scenarios, not typical app submission. Often blocked in cloud egress policies.
Port 2525
Useful alternate when is blocked and provider supports it.
TLS pairing rules
-> STARTTLS-> implicit TLS-> relay use, optional STARTTLS depending on peer policy
Mismatching TLS mode is a common cause of production send failures.
How to choose the right SMTP port
- Start with provider docs and choose recommended submission endpoint.
- Confirm required TLS model (STARTTLS vs implicit TLS).
- Validate auth behavior after transport setup.
- Validate receive-side outcomes before release.
Related references:
SMTP port troubleshooting checklist
Connection fails
Check network egress policy, endpoint host, and port accessibility.
AUTH fails only on one port
Verify TLS mode and AUTH order for that port.
Works locally but fails in production
Production often enforces stricter firewall/policy controls than local dev environments.
Production validation workflow
- Validate transport and auth in Email Sandbox.
- Add release-gate assertions with Email integration testing.
- Capture message outcomes through Email Webhooks.
- Run Email deliverability testing before major sender changes.
FAQ
What is the default SMTP port?
For authenticated client submission, is usually the default recommendation.
Is 465 still valid?
Yes. It is still widely used for implicit TLS submission.
Why is port 25 often blocked?
Many providers block outbound to reduce abuse and spam relay risk.