Exchange mail conduit

From MailWasher Server

The Exchange conduit is a DLL that implements a COM interface Microsoft has defined for such mail filtering systems. This DLL's COM object is passed each message as it is moved from the initial incoming message queue (where the messages are stored when during the SMTP conversation) to the message categorizer.

Note that, in contrast to the other MTAs, this means that MailWasher Server is not involved while the actual SMTP conversation is taking place. While is the technique currently used is that Microsoft recommends, we may change the Exchange conduit to hook the SMTP events instead in the future.

The primary motivation for this change is to improve performance: by filtering out junk mail before it goes through the Exchange Advanced Queueing Engine, the mail server load will be reduced. This optimization becomes increasingly more valuable as the percentage of mail that is junk increases, which is the current trend.

One related benefit is that all the mail conduits will then be integrated into the mail flow at the same point - during the SMTP conversation - meaning features like SMTP reject could be re-introduced safely, ensuring legitimate senders are notified that their message is quarantined 'without' having to "fake it" on some MTAs by sending back bounce messages (a big no-no as that results in mailbombs whenever spammers or viruses spoof the return-path address - which is now a very common thing to do).