X.7.8 · Security or policy
SMTP enhanced status code 5.7.8
Authentication credentials invalid
X.7.8 means SMTP authentication failed because the username or password was wrong or insufficient. As 535 5.7.8 it is the standard reply to a failed AUTH login.
| Registry code | X.7.8 |
|---|---|
| Forms in use | 5.7.8 |
| Registry name | Authentication credentials invalid |
| Subject | 7: Security or policy |
| Permanent or temporary | 5.x.x permanent failure: resending unchanged will not succeed |
| Associated basic codes | 535, 554 |
| Defined in | [RFC 4954] |
What the registry says
This response to the AUTH command indicates that the authentication failed due to invalid or insufficient authentication credentials. In this case, the client SHOULD ask the user to supply new credentials (such as by presenting a password dialog box).
IANA SMTP Enhanced Status Codes registry, X.7.8
The basic codes it travels with
- 535 Authentication credentials invalid
- 554 Transaction failed (Or, in the case of a connection-opening response, "No SMTP service here")
In practice
535 5.7.8 is the reply most people see when an app or printer can no longer send through their mailbox. Common causes are a changed password, 2-step verification requiring an app password, and providers turning off basic authentication. Microsoft 365 is retiring basic authentication for SMTP AUTH, so older devices may need OAuth or a different sending method.
What the sender should do
Re-enter the credentials, check whether the provider requires an app password or OAuth, and check that SMTP AUTH is enabled for the account.
Would verifying the list have prevented 5.7.8?
No
This reply is about your sending setup, the message or the receiving system, not about whether the address exists, so verifying the list will not prevent it.
Common questions
- What does 5.7.8 mean?
- X.7.8 means SMTP authentication failed because the username or password was wrong or insufficient. As 535 5.7.8 it is the standard reply to a failed AUTH login.
- What should I do about 5.7.8?
- Re-enter the credentials, check whether the provider requires an app password or OAuth, and check that SMTP AUTH is enabled for the account.
- Would an email verifier have caught 5.7.8?
- No. This reply is about your sending setup, the message or the receiving system, not about whether the address exists, so verifying the list will not prevent it.
Full guides for this code
Sources
- IANA: SMTP Enhanced Status Codes registry (checked 2026-11-05)
- RFC 3463: Enhanced Mail System Status Codes (checked 2026-11-05)
- RFC 4954: SMTP Service Extension for Authentication (checked 2026-11-05)
- RFC 5321: Simple Mail Transfer Protocol, section 4.2.3 (reply codes) (checked 2026-11-05)
Registry and provider wording is quoted exactly; guidance is written by the SimpleVerifier Team.
SimpleVerifier Team · Last checked against the sources 2026-11-05