I think the problem was this guy was providing email accounts for other people. Other people who probably reused their passwords and had their email in the same DB in the apps they used. That DB was compromised. Hackers got an account to send spam. Then the domain was blacklisted.

He alluded to this in

> At some point your IP range is bound to be banned, either by one asshole IP neighbor sending spam, one of your users being pwned