mail-do-fcc

mail-do-fcc is a compiled Lisp function in `sendmail.el'.

(mail-do-fcc HEADER-END)

Find and act on any FCC: headers in the current message before HEADER-END.
If a buffer is visiting the FCC file, append to it before
offering to save it, if it was modified initially. If this is an
Rmail buffer, update Rmail as needed. If there is no buffer,
just append to the file, in Babyl format if necessary.