Append this message to mbox file named FILE-NAME.
The details are as for `rmail-output', except that:
i) the header is output as currently seen
ii) this function cannot write to Babyl files
iii) an Rmail buffer cannot be visiting FILE-NAME
Note that if NOT-RMAIL is non-nil, there is no difference between this
function and `rmail-output'. This argument may be removed in future,
so you should call `rmail-output' directly in that case.
(fn FILE-NAME &optional COUNT NOATTRIBUTE NOT-RMAIL)