Please Assist - Unable to send email in third party desktop applications

Third part IT Support, assisting a home user - Having an issue where we're getting incorrect SMTP passwords in desktop applications when trying to send email. The password is correct as we've removed and re-added the account and we're able to receive email.

We've tried in both Outlook for Windows (and reset both that and the Mail for Windows application) and Thunderbird, using 587 StartTLS and 465 SSL. Outlook returns a 535 error (authentication rejected from the server) and Thunderbird states the SMTP password is incorrrect.

Everything still seems to work in Apple Mail in iOS.