Email problem over a postmaster server

We had two servers sending emails, at one point both servers were recieving emails but not sending, checked the RSS/MAPS database and both IP address have been spamed, spoke the ISP and suddenly one of the servers came back up, now we have changed cilents IP address to send via the server working but on cilent keeps getting a mail undelivered email from postmaster but only on certain emails but no one else in that office has this problem, can you guys help?