admins turn off relaying so that people can't offload their outbox full of spam onto their servers. They do it to keep their systems from getting overloaded with garbage, and stop people from forging mail from their network to other people. Usually open relay's end up in the blackhole lists which would mean that nobody would accept mail from that server after it hits that list. Meaning that that person/company would no longer be able to host their own smtp server...

so, don't go looking, and if you do and find somthing, let that company know that it's open so they can fix it.