nnmail-split-abbrev-alist

nnmail-split-abbrev-alist is a variable defined in `nnmail.el'.
Its value is

((any . "from\\|to\\|cc\\|sender\\|apparently-to\\|resent-from\\|resent-to\\|resent-cc")
 (mail . "mailer-daemon\\|postmaster\\|uucp")
 (to . "to\\|cc\\|apparently-to\\|resent-to\\|resent-cc")
 (from . "from\\|sender\\|resent-from")
 (nato . "to\\|cc\\|resent-to\\|resent-cc")
 (naany . "from\\|to\\|cc\\|sender\\|resent-from\\|resent-to\\|resent-cc"))

Documentation:
*Alist of abbreviations allowed in `nnmail-split-fancy'.

You can customize this variable.