Nope.
"I wonder if they are getting hit because all their shared clients send emails via only a few ip addresses?"
Nope, it (at least used to be) absolutely routine among some internet providers here in the US to completely block port 25 (except to their own mail servers), forcing all outgoing mail to go through their mail servers. ISP with 100,000s of customers, all mail coming out of a handful of IPs. The online time I heard of one getting blocked was when one of these ISPs decided they could make extra money by not enforcing their spam policies; when the whole E-Mail system dropped dead (combination of overload from the spammer's piles of spam plus being blacklisted), they did quickly decide maybe they should enforce that policy and booted the spammers back off.
