mail-default-directory

mail-default-directory is a variable defined in `sendmail.el'.
Its value is

"~/"

Documentation:
Value of `default-directory' for Mail mode buffers.
This directory is used for auto-save files of Mail mode buffers.

Note that Message mode does not use this variable; it auto-saves
in `message-auto-save-directory'.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 22.1 of Emacs.