Talk to SmarterMail support (or sales) about your requirements, then to your vendor.  When SM calls an external product like Declude, it creates a file pair and drops it into a handoff folder (The .HDR file contains some extracted metadata, and the .EML file contains the raw message.)   An external product is expected to pick up the file, do something with it, and then return finished messages to a different folder.  You or your vendor should be able to write code to do the pick up and drop off.

I use this feature only for inbound messages from the Internet (unauthenticated SMTP).   You need to check with SmarterMail to see whether it can handle all client protocols (SMTP, MAPI, EWS, EASE), as I am a little fuzzy on that aspect of the product.