New Store change in RU8 to reduce chance of Back Pressure on Hubs

Exchange 2007 SP1 RU8 has released to public this week and I wanted to discuss a great new change added to Store.

Maximum message size limits in Exchange Server 2007 can be set in a number of places, such as on the TransportConfig object, or individual user mailboxes. This helps prevent clients from submitting messages that are too large. One caveat, however, is that older Outlook clients (Outlook 2003 SP1 and older) could still submit a message that was larger than the maximum message size, the store would identify that the message was too large and NDR it, attaching the original message to the NDR. There are work arounds for this, such as blocking older Outlook clients from being able to connect to the store, but this isn't always acceptable in some customer envrionments and isn't a full proof way to block this issue from happening. Any custom MAPI application (regardless of the version of MAPI being used) can completely ignore that a maximum message size is present and simply submit a large message.

The problem attaching the original message to an NDR is that if the message was REALLY large, Hub Transport would often go into Back Pressure attempting to process the NDR, take note that the Hub NEVER sees the originally submitted message, only the NDR. Additionally, if it took longer than 5 minutes for the Hub to process the message, the Mail Submission Service could notify another Hub to process the message, causing a bad cascading effect through the Site.

We introduced a change in the store behavior with RU8 that causes the store to skip attaching the original message to the NDR if the cause of the NDR was due to exceeding the maximum submission size. This will greatly reduce the chance that a Hub will go into back pressure due to an excessively large NDR, assuming you have reasonable message size limits to begin with.