nnmail-split-fancy-with-parent-ignore-groups

nnmail-split-fancy-with-parent-ignore-groups is a variable defined in `nnmail.el'.
Its value is

nil

Documentation:
Regexp that matches group names to be ignored when applying `nnmail-split-fancy-with-parent'.
This can also be a list of regexps.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 22.1 of Emacs.