When a database fails over to a cross-site database availability group (DAG), the Microsoft Exchange Server 2010 Hub Transport server cannot deliver messages to a mounted database, and the mailbox delivery queue on the Hub Transport server may begin growing with deliveries destined to the failed over database.
This issue occurs because the LastMountedServer value and CurrentlyMountedServer values diverge for a long time.
To resolve this issue, install Update Rollup 8 for Exchange Server 2010 Service Pack 3.
To work around this issue, use one of the following methods:
Restart the Microsoft Exchange Transport Service on Hub Transport server.
Open Exchange Management Shell, and then run the following cmdlet on the Hub Transport server:
Retry-Queue submission-queue -resubmit:$true
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
For more information about the Retry-Queue cmdlet, see general information about Retry Queues.