Email Question - OT

[S. Bob]

Hi All;

This is probably Off Topic, apologies...


I recently kicked googlemail to the curb in favor of a service that does 
not read my emails (fastmail). We are very happy with fastmail. We are 
using our own domain which I was also doing with google.

All of our clients can now email me at the same email address as before 
and I get the emails via fastmail, however we have one client that I do 
not receive the emails for. They can email  me at other addresses but 
not the 'migrated' address. I'm not sure how to even start debugging this...

Any thoughts, ideas, suggestions would be much appreciated


Thanks in advance