Mail app is giving me a sign in error unable to signin with username and password

Since your webmail login is fine, then it's definitely not an email/password issue on your part. It sounds like they "keychain" entry that the Mac has saved for your incoming (IMAP or POP) mail account has gotten corrupted recently. It also sounds like the outgoing (SMTP) keychain entry has been corrupted for the past 2 months.

Usually, simply (or "simply" since it can be a little intimidating) opening up the Keychain Access App (via Spotlight search on your Mac), clicking the "Passwords" category on the bottom left and finding the two associated entries (they should both will have a blue circle @-symbol) and clicking on them and deleting them will clear this up and then you'll be prompted for the passwords and the Mac will be able to appropriately save and use them. If you can't find any you can search for "smtp" and it should filter the list for your outgoing mail saved password. You can also try searching for "imap" or "pop" for the incoming mail saved password entry to delete.

In a browser log into your WebMail for a test.

Have you changed your eMail password 2 moths ago?

Works sometimes even if you haven’t

Open Keychain Access in Utilities, then either check the Password under that item, change it, or delete it and start over.

Enter the part after the @sign in the search bar.

You may have duplicate or more entries in Keychain Access.

If using a browser to login via WebMail works it's not Name or Password, but likely old passwords interfering.

There''ll be at least 2 entries, maybe more.

The trouble is that Mac Mail almost always chooses the old Passwords if you don't change or get rid of the old ones.

Restart to clear some caches.

The big bugaboo with Mail is finding the keychain for any auto setup Mail accounts, if IMAP it usually involves deleting the account & setting it up again to fix it.