KB01120810 Creating ASCII files from incoming mail instead of routing it
Create a new mailslot, enable its gateway and choose your disk location for the generated files. There are a number of format options available so experiment to decide which suits your needs best.
You can direct all mail to that mailslot, or direct each existing mailslot to a different folder if you wish. Use a simple script triggered on inbound to do the address rewriting if you choose the former solution, ...
| SetHeader("X-Mtq-Rcpts", MsgGetRcpts());
... where "mailbox" is the disk gateway mailslot, because it enables you to add a header of your choice containing the original recipient(s) in case you need to recreate them later.