gnus-emphasize-whitespace-regexp

gnus-emphasize-whitespace-regexp is a variable defined in `gnus-art.el'.
Its value is

"^[ 	]+\\|[ 	]*
"

Documentation:
A regexp to describe whitespace which should not be emphasized.
Typical values are "^[ \t]+\\|[ \t]*\n" and "[ \t]+\\|[ \t]*\n".
The former avoids underlining of leading and trailing whitespace,
and the latter avoids underlining any whitespace at all.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 21.1 of Emacs.