Does your mail get delivered directly from your home/office IP to recipients or does it go via a relay? Many balcklist services will flag traffic from DSL/cable modems because of the potential for those systems to be spam-bot infected. It is easier for them to justblock the whole range than to actually identify the bots and the number of legit senders affected is very, very small.

If you are set up that way you should have the home email server relay through the ISP's on-net relay. That server is rarely blocked and they actually care about unblocking it when it happens as it affects all their customers, not just one guy at home.