RE: handling spam from gmail.

Wrong mailing list. You need to ask on the list for the MTA you are using (Sendmail, Postfix, &c).

Yes will ask soon at sendmail.

Actually, this sounds like a job for a custom milter, which would look at the domain name of the sending system, and reject the mail with your message. Dunno if there is one that works exactly like this.

I think also, I should have some contact coding milters. I think this could be ok for things like this spamhaus bad tld list or so.