I have looked over this forum, and found several things close, but nothing that addresses this issues in a helpful manner.

Zimbra is running, and working just fine. The only problem is receiving mail from external mail servers. I can send and receive fine internally, and send find externally. When a user sends an email from an external account (yahoo.com, gmail.com, etc) they get a bounce back saying the message could not be delivered.

We are using a spam filtering system with our ISP. All mail sent to your domian is first sent to their mail server (mail.isp.com) and then passed on to ours. The error comes up when their mail server (mail.isp.com) tries and handoff the mail to us.

I have spoken with the ISP on this, and they are saying that my mail server (zimbra I assume?) is refusing to accept the mail from their mail server.

My question:
How do I setup Zimbra to accept incoming mail from their mail server?

Thanks for any help.