Hmm. The error is straightforward - the SMTP server doesn't recognise the user id and password you're sending to it. This can be the error returned by "this user can log in, but doesn't actually have the permission to send" sometimes too.
I think we'd like to see more of the SMTP error (should be in your logs, but if you save the email server and then try "send test email", there's usually enough information there and it's quicker than digging through the log), but also check on the mail server side to see more detail on why it's rejecting you!
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