Many of the answers here helped point me in the right direction but not fully addressed the problem. The actual answer is the comments:

I put my actual email as Username and From and it worked on the spot. Are you still getting the same error message?

_{Thank you @user573431}

The problem here is that I was sending from any email that was not my own. The From and ReplyTo must be your email registered in SMTP.js. Hope this helped someone out!