Among the many things that I look after, I manage the email for several hundred domain names. A large portion of these domains are for individual artist websites and thus have only a couple of actual email addresses. In most cases I just forward any inbound email to the artist's ISP or web mail email account. We don't filter that email in any way so everything, including spam, gets forwarded to the owners actually email address. We have avoided spam filtering because most people already have spam filtering on their email account so our filtering would not be beneficial for the recipient. I'm sure that the spam filtering efforts by GMail, Comcast, Yahoo, etc is much better that I'm going to be able to implement.
Over the last month or so we've started running into issues with Comcast and AT&T blocking all email from our servers due to the fact that they receive what they consider spam from our servers. We have gotten our servers unblocked
but today, Comcast has blocked us again. So, to be able to deliver email to Comcast we have to "clean" all email that passes through our servers. We have no idea about what the triggers are for Comcast to block a server. The barrier is likely to be fairly low as we don't have all that much email traffic in total. So to keep our standing with Comcast, we will have to be brutal. We will have to consider any email that might possibly be spam as spam and bounce it. If only a tiny percentage of spam gets though our filters, we might get blocked again. The net effect is that some legitimate email will bounced.
While neither us nor our customers are doing anything wrong, Comcast is forcing us to not just tag potential spam as spam but forcing us to block it entirely. Essentially they are pushing their problems on us.
The net effect of all this is that Comcast will be forcing many smaller operations that process smaller amounts of email to find their own solutions to deal with the "Comcast" email problem. Each operator will find a way that will cost in aggregate thousands, maybe millions of man-hours of effort and will at a net, reduce the percentage of successful legitimate email deliveries. Spam has made email less useful but these efforts by Comcast will be adding some of the last few nails to the email coffin. I'd love to see email disappear but it won't until something better takes it's place.