What to do
- If AUTH stops here, the client is sending the wrong mechanism or credentials format.
How it appears in a bounce
334 Server challengePaste the full bounce into the decoder — it finds every code and recipient and says what to check.
SMTP 334 Server challenge
An AUTH step: the server waits for the next part of the login (base64).
334
- Class
- 3xx · Intermediate
- Retried?
- Not needed
- Defined in
- RFC 5321