Email alias problem

Last year, I created an alias to use for a specific purpose, and it initially worked fine. Today I discovered that I am no longer receiving mail sent to that address. The sender receives a delivery-error message that says "Diagnostic-Code: smtp; 550 5.1.1 unknown or illegal alias". And yet I can still send messages from that alias! And my other two work just fine.

Any suggestions? Thanks.

(Btw, I wasn't sure which community to put this in, so I'll also listen to suggestions for where else to post this.)