Interesting, I wasn't aware of this happening. Unless they had a setting to disable it, my initial reaction is that I'd disagree with those email providers bloc

Interesting, I wasn't aware of this happening. Unless they had a setting to disable it, my initial reaction is that I'd disagree with those email providers blocking domains/IP addresses too. I do think, though, that there is an easily identifiable difference between blocking obvious spam sites, likely based on autonomous algorithms, and (I presume manually) blocking a site towards which you clearly have ulterior motives.