In a multiserver Exchange 5.5 environment, you may occasionally receive error messages in your Application event log. If these errors occur often enough, you may also find that mails are backing up in the MTA queues. Here's an example:
Event ID: 9318
Category: Interface Description
An RPC communications error occurred.
Unable to bind over RPC. The locality table (LTAB) index is 76.
Windows NT error code: 1722. [BASE IL MAIN BASE 1 500] (12)
Event ID: 9322
An interface error has occurred. An MtaBindBack over RPC has failed. Locality Table (LTAB) index: 221, NT/MTA error code: 1722. Comms error 1722 Bind error 0, Remote Server Name XXXXXX
Looking up Windows NT error 1722 by using net helpmsg 1722 returns "RPC server is unavailable." And pinging the remote server by name may return inconsistent results.
Intermittent network outages can cause this behavior. However, you can normally identify network outages rather easily. In most cases, if you receive these error messages when there are no network outages, name resolution problems are typically the cause.
After Exchange 5.5 Service Pack 3, Microsoft changed the MTA so that the fully qualified domain name (FQDN) of the remote Exchange server must be resolvable to send mail. One way to test this is by entering the FQDN of the remote server in the HOSTS file of the Exchange server. If the problems go away, you know that it is indeed a name resolution issue.
While you can leave the HOSTS entry as a workaround, it's a better idea to fix your DNS problem. It may cause other problems for your network.