AuthSMTP SMTP port guide
SMTP Port 587: STARTTLS Email Submission
Use port 587 with STARTTLS for standards-based authenticated email submission. The connection starts as SMTP and upgrades to encrypted TLS before authentication and message transfer.
Quick answer
- SMTP port
587- Connection security
- STARTTLS
- Authentication
- Required
- AuthSMTP hostname
- Use the server supplied to your account, normally
mail.authsmtp.com
Do not select implicit SSL/TLS for port 587.
What SMTP port 587 means
Port 587 is the standards-defined message submission port. RFC 6409 separates authenticated message submission on port 587 from server-to-server SMTP relay on port 25.
STARTTLS begins with a normal SMTP connection and then upgrades it to TLS before credentials or message content are sent. RFC 8314 recommends supporting both STARTTLS on port 587 and implicit TLS on port 465 during the transition to encrypted submission.
Using port 587 with AuthSMTP
AuthSMTP accepts authenticated submission on port 587 with STARTTLS. If your email application offers separate SSL/TLS and STARTTLS choices, select STARTTLS.
Use the exact SMTP hostname and credentials shown in your AuthSMTP Control Panel. The hostname is normally mail.authsmtp.com.
For application-specific screens, see the AuthSMTP setup guides. If a connection succeeds but mail is rejected, follow the troubleshooting guide to check credentials, account status and authorised sender addresses.
Troubleshoot port 587
- Confirm the security mode is STARTTLS, not implicit SSL/TLS.
- Make sure authentication is enabled and the AuthSMTP username and password are entered exactly.
- If STARTTLS is unavailable or the connection is interrupted, check for firewall, antivirus or mail-gateway interception.
- Test whether the current network permits outbound port 587.
- If the application cannot use STARTTLS, try port 465 with implicit TLS instead.
A successful port test only confirms that a connection can be opened. It does not validate the account password, sender authorisation or the application configuration.
Compare SMTP submission ports
| Port | Security mode | Typical use |
|---|---|---|
| 465 | Implicit TLS | Secure submission where TLS starts immediately |
| 587 (this guide) | STARTTLS | Standards-based authenticated message submission |
| 2525 | STARTTLS | Provider-specific alternative when standard ports are unavailable |
Port 25 is primarily used for mail-server delivery and is frequently restricted on access networks. For an email application or website submitting mail to AuthSMTP, prefer 587 with STARTTLS or 465 with implicit TLS; use 2525 with STARTTLS when an alternative is needed.