[exim] How to block at SMTP time domains that are not in my …

Hello List,
would you be very kind to share a code on how to check at SMTP delivery
that the domain that is sending an email exist in the server?

I have seen some hackers that managed to know the password from an account
and then at SMTPATUH they send emails using a domain name that is not in
the server.

Or if you have a code where a mail that contains more than 10 AOL or YAHOO
addresses could be deleted and not send?

Thanks a lot in advance.

Regards,

Sergio Cabrera