It's been extremely effective in blocking spam sent by spambots on large ISPs. It's not a magic anti-spam bullet. (If you know one, please let us know.)That simply hasn't been my experience. I still get lots of spam from booted hosts in large provider networks, and yes, that includes many that block 25. As near as I can tell, 25 blocking is not affecting spammers at all, just legitimate users.
I know people at large ISPs with actual data. Port 25 blocking is quite effective.
R's, John
