First, a quick thanks to everyone that responded. I've received useful and excellent info from everyone.
We do not block on 25 outbound/inbound, but we are considering it for the residential broadband connections - maybe filter, proxy, or at least monitor it.
I should clarify one thing: We are considering REQUIRING SMTPAUTH for all connections from customers for relaying - whether they are on our IP space or not. I know this will only buy us a few months until the next round of viruses steal username/pass, but even then it will give us the ability to detect an infected/SPAMMING customer quicker and auto shut them down (vs having to shutdown the IP, and then the customer receives a new IP...)
My question is: Have any or many of the larger ISP's gone the route of REQUIRING all customers to use SMTPAUTH - regardless of where they are connected. Can anyone disclose who these regional or national providers are?
Thanks again
--Dan