550 Administrative prohibition

Hi all,

I have a setup where my smtp spam filter is on one UTM - and general firewall is another machine.

I route mail from firewall to spam filter to mail server. Fine for normal inbound.

For spam inbound:

1. Mail passed by firewall to smtp proxy
2. On spam filter I get: SMTP connection from (MAIL*) [*.*.*.*]:59851 closed by DROP in ACL
3. On the firewall the proxy will attempt to reply to the spammer based on:
   1. ...R=static_route_hostlist T=static_smtp: SMTP error from remote mail server after end of data: host *.*.*.* [*.*.*.*]: 550 Administrative prohibition
4. So effectively I end up with a spam reputation, since I reply to spam addresses

How can I prevent that?

Normally smtp would end on "closed by DROP in ACL"

Best regards