Add a stanza to the beginning of the routers section (order is important, as the first router to match is used):
driver = redirect
domains = ! example.com : ! example.net : ! example.org
data = :fail: Mailing is only allowed to example.com, example.net and example.org
This router will match other domains than those listed due to the negation exclamation marks, the
:fail: tag indicates that this is a failure.
no_more directive stops exim trying other routers for the matched domains.