SMTP Error 535 sending emails from webmail

[#1]

Logged into plusnet mailbox using webmail.  Inbox is up to date.  Try to reply to an email and get SMTP Error 535 Authentication failed.

How is this possible?

[#2]

Hi is this a permanent error? If you log out and back in again, does it still fail?

Has this ever worked?

Is it a new Plusnet account?

Has the email password been changed?

Is the the primary / default account or a secondary email address?

[#3]

Error is permanent. Still happens after logout and back in and on different PCs.

Its a very old Plusnet account and has been working for years.

Password has not been changed so should not be out of sync.

Other error 535 posts suggest that the mailbox might be blocked from sending and will need plusnet support to investigate.