As I said I’ve been wrestling with Exim for the last couple of days. The mess started when I started as MX for a new domain with primarily forwarding to a couple of external addresses.
The problem occurred because the server the mails were forwarded to checked the domain in the sender address. My current Exim setup didn’t do this, and thus accepted a bunch of spam mail which it tried to forward to the external server. No surprise, it got rejected. But because the domains were false I got a hughe queue with bounce-messages that could not be delivered.
So I decided, what the heck I could turn sender verification on too. It’s not like I have enormous loads, so I could handle the overhead. Bad idea. 😉 It seems the sender verification just checks for A-records. Domains that have no A-record, but have MX records were getting rejected. Not very popular when one of my users got a mail bounced from a business associate. 😉 So stay away from the option if you’re running Exim.