mail-alias-file

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

nil

Documentation:
If non-nil, the name of a file to use instead of the sendmail default.
This file defines aliases to be expanded by the mailer; this is a different
feature from that of defining aliases in `.mailrc' to be expanded in Emacs.
This variable has no effect unless your system uses sendmail as its mailer.
The default file is defined in sendmail's configuration file, e.g.
`/etc/aliases'.

You can customize this variable.