[Exim] Simple Question

Quick question, is the following valid ?

    sender_verify_hosts = !*.mydomain.com

I see no mention of using the ! operator in a 'host-list', but exim does not
compain in the about it in the config file. However it does not seem to be
working as I want, i.e. it seems to be matcing the ! character as part of
the domain name.

Thanks.