No-one likes spam, but I've noticed that at least some email providers do this by domain/ip address of the emailed link in order to combat spam. Maybe its different since it ends up in your spam folder, but its effectively the same result.

Details: I've had a website that got hit once by this, which was funny because we never even sent any email. When testing it out, it seemed to be being filtered by the ip address the link resolved to. Even if I changed the domain name I emailed, if it resolved to the same ip, it still was being marked as spam. Note that this was being tested from a 3rd party email, not by sending from our own mail server.

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.