Outgoing Mail No Working

For some reason my Mail app quit sening mail. It bounces back and forth ad nauseum until I finally delete the message. I CAN send email from the browser version, but prefer to use the mail app. This also occurs on my iPad so it is a question.

Secondly, my Apple ID is now not correct apparently. How do you reset it? This is on a second email account and unrelated to the primary account.

Thanks.