KwikPhix,
Is this too easy a solution, but why don't the ISP's scan your e-mails before they send them on ? Surely this would be to everybodies benefit. If I was given the option of having my e-mails scanned whilst they where sitting in my ISP's server prior to me 'picking them up' I would go for it. Somebody like a large organisation like BTopenworld could probably afford to get AV software from all the major AV software developers and use them all at the same time to cover all angles.
It's all down to cost. We'll assume that a big ISP will get a good deal on the software, but it still takes a lot of CPU, disk bandwidth and memory to scan large amounts of email for viruses, all of which has to be bought and kept running. IPSs are as cost conscious as any other business and will have to way up the
PR benefits from doing it with the very real costs...