500 5.5.6: authentication data too long
During login, the client sent an authentication response longer than the server can accept for the chosen method, so the AUTH command failed.
- Standard
- Provider
- Permanent (5xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 500 5.5.6
- Provider
- Standard SMTP codes
- 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
500 5.5.6 Authentication Exchange line is too long
What it means
RFC 4954, which defines SMTP authentication, added X.5.6 for one case: the client's base64 response in the AUTH exchange is longer than the server's buffer for the selected mechanism. The RFC pairs it with reply code 500 and notes it can be permanent or temporary; in practice it reads 500 5.5.6. It happens at login, before any message is sent, so it affects every message from that client until it is fixed. Long credentials, such as OAuth access tokens, are one way to reach the limit.
Common causes
- A very long credential or token sent in one AUTH response.
- A server with a small buffer for the selected mechanism.
How to fix it
- 01Try another authentication mechanism the server offers.
- 02Update the mail client or library, which may handle long tokens differently.
- 03Check that the token or password is the one you expect; a corrupted value can be much longer than normal.
- 04Ask the server's administrator whether the limit can be raised.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
Related codes
- 500Command not recognised.The server did not recognise the command it received, or the command line was too long. This is a permanent error in the SMTP conversation.
- 535Authentication credentials invalid.The server rejected the login because the username, password or token is wrong or not sufficient for this account, so no mail is sent.
- 535 5.7.8Login failed: invalid credentials.The SMTP server rejected the login because the username, password or token was wrong or not sufficient, so it will not accept mail from this session.
- 5.5.2SMTP syntax error.A command from the sending software could not be understood, because its syntax was wrong or the command was not recognised at all.
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