How difficult would they be to port?
Not too difficult, I guess. NT has a POSIX compliant environment?
The standard make process might not work, because I believe it's only
POSIX.2 compliant? (libc level).
But if I understand correctly, the POSIX features are such that when
you use them, you cannot use any of the other features of the operating
system. So to do a real port, it would really need to use native NT calls
rather than POSIX calls.
I suspect that it would be a fair amount of work to correctly port
procmail to NT. To give you an example, I've seen an ad by a company
selling a port of sendmail to NT. I would think porting procmail (and
SmartList) to NT is a good commercial opportunity for someone (Stephen?).