Two months ago a client of mine was required to shift their domain hosting to their parent companies servers, however I still handle their POP3 emails, the MX records still being through me.

One user has not received any emails known to be sent from anyone using the parent companies domain since Feb 25th.
All other emails are coming through.
No other users are experiencing problems.
Prior to Feb 25th, emails from the parent company were received.

That the problem is so specific is leaving me

Does anyone have any idea as to what the problem could be?