We're looking into implementing Variable Envelope Return Path (VERP) for improved bounce processing for our application.

Our current mail infrastructure is MS Exchange 2007 but are in the process of upgrading to 2010. We're also implementing Postini for spam filtering.

Exchange doesn't support sub-addressing (see also this question on disposable addresses) -- and VERP is somewhat of a specialized application of sub-addressing.

Are there any options for implementing VERP in Exchange without putting another non-Exchange SMTP relay in front of Exchange to pre-process incoming messages? Specifically could a transport rule be created that could match against the target (non-existing) recipient, store that recipient address in a special header added to the message, and redirect the message to a pre-created mailbox?

Note: we have developer resources available if custom code could be used somehow.