Our Exchange 2000 Server computer queues mail when we specify our ISP's mail host with the Forward mail with unresolved recipients to check box, and the queues never empty. If we don't use this option, mail flows normally, and mail sent directly to the ISP's mail server works fine. What's the problem?

This problem occurs when you have Messaging API (MAPI) clients sending to your Exchange 2000 server. Recall that MAPI clients don't use native Internet message formats and that Exchange 2000 doesn't convert between its native format and the Internet MIME format unless it's absolutely necessary. If a MAPI client submits a message to Exchange 2000 and that message must be forwarded, Exchange sends the MAPI data as a binary blob after sending the SMTP DATA verb. That process works if your Exchange 2000 server is talking to another Exchange 2000 server; however, if you specify an Exchange Server 5.5 server or a third-party SMTP server (including sendmail and qmail—the two packages that ISPs use most frequently), guess what? The remote server chokes on the binary blob, the message transmission fails, and the messages stay in your queue. This problem is a known gripe against Exchange 2000. The Microsoft article "XADM: MAPI Messages Stack Up in Send Queue to the Host Specified in Forward Unresolved Recipients" (http://support.microsoft.com/support/kb/articles/q275/5/96.asp) acknowledges the problem and explains where to obtain the hotfix. Microsoft recommends that you install the fix only if this problem "severely affects" your server. If not, wait for Exchange 2000 Service Pack 1 (SP1). As a workaround, clearing the Forward mail with unresolved recipients to check box will do the trick, although that action will affect your mail-routing topology.