User will send a request saying they are not receiving emails from one client. All other emails work fine (They can even send to that client).
Login to their computer.
Have them login to their webmail and go to Settings => Options => Mail => Blacklist.
More often than not, that persons email will be in there. How it gets there, I don’t know. Many claim it’s a glitch (Google it) but by removing it, the issue will be resolved.