Most things with an executable attachment we bounce, in case they're viruses, with a message saying to zip it up and try again if they really wanted to send it. Klez and various other viruses forge the sender though, and we don't want to go round sending bounces to forged senders who will either be confused about it or will just bounce back again. So we want to try and freeze any of them that actually look like Klez. So gradually our system filter turns into not-very-good virus detection software. It really would be better if we could find the manpower to sort out a real one instead.
Anyone who's actually using something with exim already do feel free to let me know what it is and how easy it is to set up :-) I think we investigated something in the past but it fell down because we couldn't get it to send bounces as the error sender, this is before my time though.
