How exactly does Sitefinity calculate the delivered status f

I have a client that sent a campaign blast out last night to 9,123 subscribers.  Only 958 were delivered, but it shows that 9,123 were sent.  When checking the server badmail que and event logs I could only find about 75 sitting around for various reasons, but certainly not the 8,165 that I am trying to account for.

Does the "sent" status just grab the number of subscribers from Sitefinity or does it check how many actually made it from Sitefinity to the servers mail que before reporting?

Is it possible that these messages might have been sent, but something would have caused the Sitefinity service that monitors report tracking to not update correctly?

What I read about the "Delivered" status is that it accounts for messages that may have gone to recipients that do not have valid addresses.  Does it pick this information up directly from the server when messages are put into que or is it possible that some messages were sent from the server, but caught by their destination servers spam filtering and considered not delivered?

This client sent out a separate campaign to the same list of subscribers last month with a delivery stat of 9,044 out of 9,123 so I would find it hard to believe that suddenly that many of them were bad addresses.

Any suggestions on how I might track more information down on this would be great.

I noticed I am missing a web.config section that was introduced for upgrading 5.2 -> 5.3 (I did this before those documents were updated) that might have bearing on this....hopefully.

I will be trying to run this campaign again later tonight and see what happens.

We found an error message in our Trace.log that helped point to a solution.  

Turns out the new campaign was hitting a session limit with Windows 2008 smtp server.  We significantly increased the session limit setting found under messages and were able to get all the messages to deliver.