Authentication failed: 535 Incorrect authentication data
I just ran into this trying to use SquirrelMail. It also affected RoundCube and likely Outlook. Usually this feels like a permissions problem server wide, but this only affected one account. One that had recently changed servers.
Incorrect authentication data
A search gave me many solutions, many complicated root-based commands. And one simple solution. I like simple.
Odd, but it worked. Even though both servers have the same setup, moving from one server to the other appears to have confused something along the way.
User is able to login to webmail just fine. And receiving is no problem. But when trying to send mail, either new or replying, we just received errors. It looks like the emails was actually delivered, but you'd never know it because of the error. And I read that if Outlook keeps asking you for the password and you are positive it is correct, a password reset fixes it.
- Log into your cPanel Webmail
If you have it automatically open your favorite mail program (like squirrelmail or roundcube), click Stop to stay on the settings page.
- Click 'Change Email Password'
- Enter same password, or use this opportunity to create a new, stronger one.
If you use the same one, you might get an error saying 'account already exists'. Don't worry, just ignore it.
- Now, go to the mail program and see if you can send.
Hopefully all works again. If not, make sure you note the exact error message and contact your server admin. But they will have to do some of that fancy, magically, root command stuff.