[exim] Force valid sender domain from php

Can we restrict mails sent from the local host to only be allowed to
have their sender/from address be from a valid domain in /etc/localdomains?

If a php script is used by a spammer he will usually set the from
address to something other than the real domain e.g.
accounts@???. As paypal.com in not listed in localdomains can we
reject it?

I hope I am making sence. Thanks in advance.

Will