Emails Rejected

I have a client who had a virus attack and emails from thier server were rejected by quite a few other servers (ISP’s?).

Since my email address is the only one being rejected, the client suggested that I call my ISP, which is Ameritech, so they can remove any restrictions. I contacted my ISP and they indicated that since my email account is not corrupted and I can send and receive all other emails, that there must be a server who sits between me and the client who is rejecting the sent emails.

Can anyone shed some light on what might be happening? And the solution, please.