Solution:
I think I solved it.
Failed to authenticate SMTP server with "email" using two possible authentications.
So, I was playing around and going crazy with this issue. This made me nuts and may make you or any one this forum as well.
The first thing was to understand the error.
The meaning of the error is that the SMTP server you are trying to verify is being verified by the same email id twice or there are two email ids using the same password to verify the server. This is the reason the error occurs.
So if you are using username and password, and e-mail (which is required) this error is most likely to come because you will be using username and e-mail for the authentication for the SMTP server.
Solution:
Remove the email from username and just keep the e-mail in the e-mail section which is required and let the password be in the password section. You should be good to go now. If you still facing the issue, see the guide present at:
https://kb.mailwizz.com/articles/fa...username-xyz-using-2-possible-authenticators/
The another issue can be of the password or firewall ( if you are using it)