Same problem here, and my problem is that over 50% of my contacts have the same email provider (our local school district). I was given a list of IP addresses to put in our system, but that didn't help. I am now trying to find another company like this since having over 50% of my emails get bounced as non-existent address (including MINE!) makes having this whole thing pointless and a waste of money.
Oddly enough, we can send to one user at a facility but not another. The problem seems random on specific domains but occurs on the same email address consistently. Do you think it has something to do with CaSe SeNsITivity?