Please post details of exactly how the account was migrated, plus complete error messages, plus screenshot of account server settings and SMTP server settings. Thank you. All of that may prove helpful in identifying the problem.

Used your migration instructions. And uncertain on the USer Name...for my provider or for Mozilla: Thunderbird?

For your provider, and almost certainly not what you have in the picture.

User names or login names are usually either the part of the email address before the @ or the email address itself. But really that is only a best guess. What your provider chooses to use/accept it entirely up to them.