I have a client who is using a bit of a strange setup. He receives mail directly via SMTP, it is first received by his sophos firewall SMTP proxy. The firewall calls out mail that is received to verify if the user exists before accepting the message. Messages are then scanned and forwarded to the mailsserver.
Mail is sent, however, via a smart host of a different provider.
(Long story - it has to do with their line issuing fixed IPs from a DHCP pool, so they are not fit for running a mailserver on because sent mail will not be accepted by many providers).
This provider is then sending Mailer-Deamon replys to also non-existent senders (spammers) which results in blocking the account after a while.
How do I train exchange to tell the firewall that an address does not exist so that it is rejected before transmission?
To those who know Sophos: The firewall is AD-member, but setting it to check for existing adresses in AD results in each and every mail being rejected because it does not exist even though it does.