You have multiple Microsoft Exchange Server 2007 hub servers that use the DNS round robin feature. If one hub server is taken offline, e-mail messages for that server are queued. The e-mail messages are processed by other hub servers after several hours.
Note An environment may contain as many as 4000 queued messages.
Exchange Server 2007 maintains a list of hub servers and their states. The round robin feature selects hub servers that have an available status. Unavailable servers are retried every five minutes. However, when these servers are retried, they are marked as "Active," even if they are not available. This behavior causes e-mail messages to be continually submitted to unavailable hubs. Therefore, the e-mail messages are queued.
To resolve this problem, apply the following update:
971534 Description of Update Rollup 1 for Exchange Server 2007 Service Pack 2
Note This fix does not fix all message queuing issues that are caused by the retry logic. It addresses only the issue described in the "Cause" section.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.