We have several custom fields created that could be changed to certain values based on rules (sender's email, sent-to address, etc), but a Business Rule only allows me to manipulate the built-in fields. As a workaround I have made several incident templates that have the values populated appropriately, and then I have Business Rules to apply those templates, but that gets really messy and it doesn't work when there are multiple fields that would need to be updated.

Thanks for your consideration, and let me know if you have questions!

Colin