We have the same issue. Online surveys being sent as invites to 100s of 1000s of people per day and although we are good at spotting SPAMMERs setting up free accounts, obviously people will always receive an invite they might consider SPAM and we get derated. Persnally, we find Yahoo is the worst for blocking.

It would be good to have a central registry of mail forwarders so that SPAM is not detected against registered IPs and then issues can be followed up directly with anyone who abuses the system. Do you really want to greylist Amazon SES because one of its users sends out spam?