An SMTP authentication failed error means your mail client or script connected to the outgoing mail server but the server rejected the login. You can usually still receive mail, but sending fails or the app keeps asking for your password. On cPanel hosting the cause is almost always the username, password, port or an IP block after repeated failures.
Before you start
- Find the correct settings in cPanel → Email → Email Accounts → Connect Devices for the mailbox.
- Check that you can log in to webmail with the same address and password.
Correct SMTP settings for cPanel mailboxes
| Setting | Value |
|---|---|
| Username | The full email address, e.g. you@example.com |
| Password | The mailbox password (not the cPanel password) |
| Outgoing server | As shown under Connect Devices (often mail.example.com) |
| Port and security | 465 with SSL/TLS, or 587 with STARTTLS |
| Authentication | "My outgoing server requires authentication" – use same settings as incoming |
How to fix SMTP authentication failed
- Test the password in webmail. If webmail rejects it too, reset it in Email Accounts → Manage → Security. See webmail login problems.
- Use the full address as username. Using only the part before @ fails.
- Turn on outgoing authentication in the client. In Outlook: More Settings → Outgoing Server → My outgoing server (SMTP) requires authentication.
- Match port and encryption. 465 needs SSL/TLS; 587 needs STARTTLS. Mixing them causes handshake and login errors.
- Remove saved old passwords on every device. Phones with an old password keep failing and can get your IP blocked for everyone in the office.
- Check for an IP block. If webmail is unreachable from your network but works on mobile data, your IP was blocked after failed logins. Follow unblock your IP.
- For PHP scripts and WordPress SMTP plugins, use the same full username and port 465/587, and do not set the "From" address to a domain you do not own.
Tip: If you only need WordPress to send mail, an SMTP plugin using the mailbox credentials above improves delivery and avoids PHP mail problems.
What to check afterwards
Send a test message to an external address (for example Gmail) and check it arrives. If it bounces with a 550 or 554 code, that is a different problem.
FAQ
Why does receiving work but sending fails?
Outgoing authentication is off or set to different credentials in the client.
Can I use port 25?
Many ISPs block port 25 from home connections. Use 465 or 587.
Why do I keep getting asked for my password?
The saved password is wrong, or an IP block makes every attempt fail.