[Exim] Filtering

I have a pretty straight forward request, though it is hard to put in plain
terms.

We have an email alias called vtallstaff, and is looked up in LDAP and
returned is about 1000 email addresses. We want this alias to be only
available to internal users. So that spammers cannot get/use it.

So if the email to vtallstaff@??? is originating from
159.105.102.0/255.255.255.0 then allow it.
Otherwise reject it with a bounce message saying that this email alias is
restricted to VTDOC employees.

Is this a hard request?
Can someone point me in the proper direction to get started.

PS. I would also like to limit the message_size_limit to 2MB. Can this be
done in the same filter?