We're placing a new mail gateway running MailCleaner in our DMZ, and in its default configuration it will deny invalid recipients based on the Exim RCPT ACL. We get tons of spam sent to nonexistent addresses, and so I've tweaked the Exim conf to drop rather than deny on an invalid recipient, and verified that it it will drop a connection with a 550 error when a RCPT TO is issued with a bad local address part to our domain.
I'm wondering if there's any downside to doing things this way -- I'd rather not have a spam sender be able to connect and simply try one bad address after another, and definitely don't want to accept and/or bounce messages for bad addresses. I know tar-pitting is an option, but frankly it seems like a waste of resources to keep the connection open once a bad address is entered. Is there a drawback to just dropping it over tarpit or plain deny that I'm not aware of? Thanks in advance.