The error that the other server returned was: 550 550 Mailbox unavailable or access denied – (state 13).

The error that the other server returned was: 550 550 Mailbox unavailable or access denied – <example@comcast.com> (state 13).

I was sending an old contact an email, and got this in my bounceback, and NO, the email address in my example is not the email address I was sending to.  The fix was simple enough, I had to change the address to the .net TLD and was then able to resend the message.