nnmail-split-fancy-match-partial-words

nnmail-split-fancy-match-partial-words is a variable defined in `nnmail.el'.
Its value is

nil

Documentation:
Whether to match partial words when fancy splitting.
Normally, regexes given in `nnmail-split-fancy' are implicitly surrounded
by "\<...\>". If this variable is true, they are not implicitly surrounded
by anything.

You can customize this variable.

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