550 Invalid recipient

[Joss]

certain emails don't make it through to me. above is the error message
received by those trying to email me. What is causing this and how can I fix
it. Our organizing has lost contacts and money because of this!

 

[Brian Tillman]

certain emails don't make it through to me. above is the error message
received by those trying to email me.

If they've correctly specified your mail address and they are receiving this
message, the mail router that accepts the mail for your company has a
configuration problem. Who cares for your incoming mail server?