SMTP Error 550 fix

Fast job - Need a Centos 7 expert to fix a SMTP Error on my server (see attached). This is a fast job. I will give you root access to the server to resolve. This issue is only happening on one email alias address and no other. I would like to have this resolved permininantly, for it does crop up every few months or every year on different email addresses (this one is an alias email). Diagnose, resolve, offer reasons and supply way of keeping this from happening again. Would like to have this completed today.