We have an emailhandler defined that is listening to new email and putting it in a custom repository.

But, we also have a backlog of email that was handled by another system and in a mailbox. It can be dumped as .eml or .msg files. 

I'm wondering if there's anything around that would process these emails to our email handler.

We don't want to do a forward as then we lose some of the custom headers and we rely on those for some of our processing.