message-fcc-handler-function

message-fcc-handler-function is a variable defined in `message.el'.
Its value is
message-output

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

Documentation:
*A function called to save outgoing articles.
This function will be called with the name of the file to store the
article in. The default function is `message-output' which saves in Unix
mailbox format.

You can customize this variable.