Permanent failure — do not retry
SMTP 535
authentication credentials invalid
SMTP 535 means the server rejected the credentials your client offered. RFC 4954 defines 535 5.7.8 as "authentication credentials invalid". It is about logging in to your own sending server, so no mail was attempted and nothing is implied about any recipient.
How it appears
535 Authentication credentials invalid
535 5.7.8 Username and Password not accepted
535 5.7.3 Authentication unsuccessful
What causes it
- Wrong username or password, or a password that was changed and not updated in the client.
- Two-factor authentication is on and the provider requires an app password.
- SMTP AUTH is disabled for the account by an administrator.
How to fix it
In order — try the first one first.
- Re-enter the credentials exactly as your provider documents them, including the full email address as the username where required.
- Create an app password if the account uses two-factor authentication.
- Ask the administrator whether SMTP AUTH is allowed for the mailbox.
Common questions
- What does SMTP 535 mean?
- SMTP 535 means the server rejected the credentials your client offered. RFC 4954 defines 535 5.7.8 as "authentication credentials invalid". It is about logging in to your own sending server, so no mail was attempted and nothing is implied about any recipient.
- Should I retry a 535 error?
- No. 535 is a permanent failure, so the message will never be delivered no matter how many times you try. Remove the address — repeated sends to permanent failures are one of the main signals mailbox providers use to throttle a sender.
- How do I fix SMTP 535?
- Re-enter the credentials exactly as your provider documents them, including the full email address as the username where required.
Specific 535 replies
Last reviewed 2026-09-11