IONOS (1&1) · permanent error
535: SMTP authentication failed
Your email program could not sign in to the IONOS outgoing server because the username or password stored in it is wrong.
- IONOS
- Provider
- Permanent (5xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 535
- Provider
- IONOS (1&1)
- Temporary or permanent
- PermanentRefused for good: the message comes back to the sender, and resending it gets the same answer.
- Who has to act
- The sender. The fix is on the sending side: your domain, your server or your list.
- What the server says
535 Authentication failure: SMTP AUTH failed with the remote server
What it means
IONOS customers send through its SMTP servers after signing in. This 535 means the sign-in failed: according to IONOS, the username or password for the IONOS email account is incorrect in your email program. Nothing was sent. A common trigger is a password changed in one place but not in every device that uses the account. Correct the outgoing server settings in each mail program; IONOS keeps setup guides for the common programs in its Help Center.
Common causes
- The password was changed but not updated in the mail program.
- The username in the outgoing server settings is wrong or incomplete.
- The mail program uses an outdated or wrong outgoing server configuration.
How to fix it
- 01Open the outgoing (SMTP) server settings of your mail program.
- 02Check the username and enter the current password.
- 03Compare the server, port and encryption settings with the IONOS setup guide for your program.
- 04Send a test message.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
Related codes
Further reading
All 17 IONOS codesEvery code in the wikiPaste your own bounce