extracting mail body (help)

Hi, 

Ive been lurking for a bit now hoping to glean some
help on this. I guess its been dealt with before I joined the list.

Anyway, I'd like to extract the body of certain incoming mails and
save them to text files with names correspoding to parts of their
subject.
i.e. a mail with
subject : <keyword> <key number> 

gets saved to text file foo.<key number>.txt

can anyone shed some light on this for me? overwriting the
text files would be better than appending to them.