message-signature-file

message-signature-file is a variable defined in `message.el'.
Its value is

"~/.signature"

Documentation:
*Name of file containing the text inserted at end of message buffer.
Ignored if the named file doesn't exist.
If nil, don't insert a signature.
If a path is specified, the value of `message-signature-directory' is ignored,
even if set.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 23.2 of Emacs.