The Messages with an unreachable destination queue contains messages that cannot reach the intended destination server. For example, Exchange cannot determine a route or a connector to the final destination. Alternatively, all available routes or connectors are marked as down. If all goes well, the following command to force a re-transmission:
Type in PowerShell:
Retry-Queue -Identity "servername\Unreachable" -Resubmit $true
Exchange Server database transaction logs record all changes to an Exchange Server database. Over time, these log files accumulate and use all the available disk space if they are not periodically removed from the hard disk.
Just a few words:
1. Dismount the Exchange Mailbox Database you wish to clean up.
2. eseutil /MH database.edb
If database are in a Clean Shutdown or Consistent state (Older ver), you may remove all the transaction logs (e000#*#.log).