mail-complete-function

mail-complete-function is a variable defined in `mailalias.el'.
Its value is

ispell-complete-word

• This variable is obsolete since 24.1;
  use `completion-at-point-functions' instead.
• This variable may be risky if used as a file-local variable.

Documentation:
Function to call when completing outside `mail-complete-alist'-header.

You can customize this variable.