This file is indexed.

/usr/include/GNUstep/GNUMail is in gnumail.app 1.2.2-1.1+deb9u1.

This file is owned by root:root, with mode 0o777.

It is a symlink to /usr/include/GNUstep/../../lib/GNUstep/Frameworks/GNUMail.framework/Headers