mail-personal-alias-file

mail-personal-alias-file is a variable defined in `sendmail.el'.
Its value is

"~/.mailrc"

Documentation:
If non-nil, the name of the user's personal mail alias file.
This file typically should be in same format as the `.mailrc' file used by
the `Mail' or `mailx' program.
This file need not actually exist.

You can customize this variable.