Strangely some ISP's also block certain IP addresses and/or domain groupings - not just ports.
Depending upon what filters/reporting they use to update their tables certain legitimate addresses are sometimes caught up in this procedure.
Even my own spam filtering at home sometimes marks very well known & legitimate emails as spam and I have to manually add them to my friends list.
Occasionally I get a group of emails from the same sender and one of them gets marked as spam while the others don't - go figure.
I am not saying this is your problem here Paul but if I were you I would hassle my ISP for a definitive answer...
Good luck.
Kind regards,
-=Glyn=-