SMTP Send Adapter, Errors and Suspended Messages Until Hosts Instance Restart

I've got a BizTalk application that has to send out several hundred, close to a thousand emails over the course of 3 or 4 hours in the morning. The app will run fine for several days, then it seems that the app will slow way down, eventually I will see all of the out going messages in the 'active' state, but not doing anything, just sitting there, with this warning...

The adapter failed to transmit message going to send port "" with URL "". It will be retransmitted after the retry interval specified for this Send Port. Details:"The transport failed to connect to the server.

I don't see any unusual load on the box, no high CPU, disk, or Network utilization.

After I restart the host instance that is hosting this SMTP send port, they all continue and run fine, for a day or two until I have this issue again.

I've been scratching my h开发者_如何学编程ead on what may be causing this issue... any ideas?

Possibly look for throttling conditions, especially for Memory throttling - (Throttling State 4) - use Perfmon or SCOM on this counter.

Also, in task manager look at the Memory of your BizTalk service hosts - and add Commit Size (i.e. including virtual). It is possible that your Orchs aren't releasing memory or are too memory intensive (e.g. remember to Dispose() XLangMessages in custom assemblies).

If you do find Throttling state 4, and are sure that you aren't leaking, you might want to bump the throttling threshold up from 25 to say 50 - see here. But IMHO 100% as suggested in the article sounds dangerous.