Hi, I'm Pete. Welcome to JustAnswer! I'm reviewing your question now, and will post back with your reply ASAP.
The "5.7.1 Authentication" message usually means that the SMTP server couldn't authenticate you as a legitimate user.
Does this happen only when you try to send email, or receive email, or both?
Who is your email service provider?
What is the second part of your email address (after the @ symbol)?
Hi, just checking in on your question, do you need any further assistance?