Choosing the right SMTP port is one of the fastest ways to fix email delivery problems.
If you are searching for "SMTP port", "default SMTP port", or "secure SMTP port", this guide covers the practical differences between ports , , , and and when each should be used.
You can access your MailSlurp account via SMTP or IMAP using the dashboard app.
What is an SMTP port?
An SMTP port is the network endpoint used for email submission or relay.
- SMTP = Simple Mail Transfer Protocol
- Port = a numbered endpoint used by client/server applications
The right port depends on your use case:
- sending from an app
- server-to-server relay
- working around provider restrictions
SMTP ports at a glance
| Port | Typical use | Encryption model | Recommended today |
|---|---|---|---|
| 25 | Server-to-server relay | Optional STARTTLS | No (for app submission) |
| 465 | Message submission | Implicit TLS | Yes, in some environments |
| 587 | Message submission | STARTTLS | Yes, default choice |
| 2525 | Alternate submission port | Usually STARTTLS | Yes, fallback option |
Port 587 (recommended default)
Port is the standard message submission port for authenticated clients.
Why it is usually best:
- Modern standard for authenticated SMTP submission
- Supports STARTTLS upgrade
- Widely supported by cloud providers and email services
For most applications, set:
- host: your SMTP provider
- port:
- TLS: STARTTLS
- auth: required
Port 465 (secure SMTP port with implicit TLS)
Port uses implicit TLS, which means encryption starts immediately on connect.
Use port when:
- your provider explicitly recommends it
- your environment prefers implicit TLS configuration
It is still common and valid, but remains the default recommendation for most integrations.
Port 25 (relay, not app submission)
Port is historically the SMTP relay port between mail servers.
Limitations:
- Often blocked by cloud networks and ISPs to reduce abuse
- Usually not intended for authenticated app-level message submission
Use for mail transfer infrastructure, not typical application sending.
Port 2525 (fallback)
Port is a common alternative when is unavailable due to network policy or provider constraints.
Many SMTP providers support it as a practical fallback for submission workflows.
How to choose the right SMTP port
Use this order:
- Try
with STARTTLS - If unavailable, try
with implicit TLS - If both are blocked, use
if your provider supports it - Use
only for relay-specific infrastructure cases
Common SMTP port mistakes
Wrong TLS mode for selected port
should generally use STARTTLSshould generally use implicit TLS
Assuming port 25 works everywhere
Many cloud platforms block outbound port . Check your network egress rules first.
Missing authentication
Most submission ports require SMTP AUTH. Without it, sends fail or get rejected.
Quick troubleshooting checklist
- Verify host and port from provider docs
- Confirm TLS mode (
STARTTLS vssecure) - Confirm SMTP username and password
- Check firewall egress rules
- Inspect SMTP response codes and logs
SMTP ports and MailSlurp workflows
For product teams, selecting the right SMTP port is only part of the job. You also need to confirm that sent messages arrive and contain valid links/content.
Use MailSlurp to:
- test SMTP sending from apps
- receive emails in code
- validate signup and reset flows in CI
Related guides:
FAQ
What is the default SMTP port?
For authenticated client submission, is the default recommendation in most modern setups.
What is the secure SMTP port?
Both (with STARTTLS) and (implicit TLS) are secure when configured correctly.
What port is SMTP for relay?
Traditionally port , mainly for server-to-server relay.
Final take
If you only remember one rule: start with port and authenticated STARTTLS, then use or as needed.
For end-to-end validation of real email workflows, create a free MailSlurp account at app.mailslurp.com.