I was lightly probing this function's behavoir.
It handles rfc882 fields:
sender - will generate using 'from' field if not in
doesn't handle rfc882 fields:
resent- I think this field may be obsolete
If there are other rfc822 fields or behavoirs for the function, then I can't speak of them as they weren't in my test.
As it is relevent to what I'm currently doing I may add more about this function from time to time.