Set up mail composition draft with the MH mail system.
This is the `mail-user-agent' entry point to MH-E. This function
conforms to the contract specified by `define-mail-user-agent'
which means that this function should accept the same arguments
The optional arguments TO and SUBJECT specify recipients and the
initial Subject field, respectively.
OTHER-HEADERS is an alist specifying additional header fields.
Elements look like (HEADER . VALUE) where both HEADER and VALUE
CONTINUE, SWITCH-FUNCTION, YANK-ACTION, SEND-ACTIONS, and
RETURN-ACTION and any additional arguments are IGNORED.
(fn &optional TO SUBJECT OTHER-HEADERS CONTINUE SWITCH-FUNCTION YANK-ACTION SEND-ACTIONS RETURN-ACTION &rest IGNORED)