mh-compose-letter-function

mh-compose-letter-function is a variable defined in `mh-e.el'.
Its value is

nil

• This variable may be risky if used as a file-local variable.

Documentation:
Invoked when starting a new draft.

However, it is the last function called before you edit your
message. The consequence of this is that you can write a function
to write and send the message for you. This function is passed
three arguments: the contents of the TO, SUBJECT, and CC header
fields.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 6.0 of the MH-E package that is part of Emacs 22.1.