, it seems you are not the only one facing this problem. I have a lot of my colleagues facing the same problem. Ok I had faced the same one few months ago and it was due to that my Microsoft Exchange Mail Submission service on the mailbox cluster was not started. It seems for some reason when installing Exchange 2007 prior to SP1 it used to fail to automatically start the submission service on the mailbox server and you have to go and change the service user to domain admin account or to a service account with proper permission and start it and enable it if disabled. I had to restart the server after that and things got to start working. That was said there is few other reasons this can happen. Below I am quoting Microsoft article for the different reasons and resolutions, but with few modifications and additions.
* The Microsoft Exchange Transport service is not running on a Hub Transport server.
* The MAPI RPC connection is blocked between the Mailbox server and Hub Transport server.
* The Hub Transport server has reached the maximum number of submission threads that the server can have open at the same time to send messages from mailboxes. This behavior may occur if the computer that is running the Mailbox server role has multi-processors and the computer that is running the Hub Transport server role is a single processor. In this configuration, the Mailbox server may submit more e-mail traffic than the Hub Transport server can process.
* not enough memory (4GB is required) or Disk Space resources.
* Microsoft Exchange Mail Submission service on the mailbox server or cluster not running.
To resolve this warning, follow one or more of these steps:
* Make sure that the Microsoft Exchange Transport Service is enabled and running on the Hub Transport server.
* Make sure that no firewalls filter network data or block MAPI remote procedure call (RPC) traffic between the Mailbox server and Hub Transport server.
* Make sure that the Mailbox server and the Hub Transport server are not deployed in a perimeter network.
* If this frequently occurs, consider whether you should add more processors on the Hub Transport server. You can also increase the value of MaxConcurrentMailboxSubmissions by using the Set-TransportServer cmdlet.
* Make sure that the Microsoft Mail Submission service on the is enabled and running on the mailbox server / cluster.
* IF you find event 15002 in the eventlog from MSExchangeTransport stating insufficient resources then the best way to resolve it is to provide sufficient resource, but if that is not feasible then you can follow the resolution below for non-production or immediate temp solution:
1- open the EdgeTransport.exe.config file in C:\Program Files\Microsoft\Exchange Server\Bin.
2- either disable exchange monitoring threshold completely or increase the threshold for the desired parameter. To disable monitoring completely you must change the following string
<add key="EnableResourceMonitoring" value="true" />
and replace it with
<add key="EnableResourceMonitoring" value="false" />
It would be much better to not disable monitoring threshold completely and only increase the threshold value for the resource that has insufficient resources most likely disk or memory. It is totally not recommended to change these threshold or disable monitoring in production. I am not sure even if that is support solution for production environment. The best resolution as I mention is to meet the hardware requirement and ensure sufficient resources.
I hope that helps.
Please post back your feedback.