Please Assist - Unable to send email in third party applications despite setting to allow

Third party 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. 

As an update to the above, we've also confirmed we are unable to send in ANY third party mail client, mobile or desktop. Password has been reset several times, and we can confirm it's correct as we're able to log into the web portal and send email.

Issue seems to be tied to either the third party email security setting not being properly toggled, or desynchronization of smtp from main password.