mail-sources

mail-sources is a variable defined in `mail-source.el'.
Its value is

((file))

Documentation:
Where the mail backends will look for incoming mail.
This variable is a list of mail source specifiers.
See Info node `(gnus)Mail Source Specifiers'.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 24.4 of Emacs.