Yes, I'm almost certain that it had been rejected by an upstream e-mail provider or even by your own ISP if either of them uses SPAMCOPs or similar e-mail server{s} blacklisting "service".
B/w - SPAMCOPs is not the worst one - they, at least, let their "victims" know that their IP addresses were blacklisted in the SPAMCOPs database. Many other "spam protectors" don't even bother to notify a network to being "blacklisted".
Unfortunately, many ISPs, organizations and even private individuals - all who have their own e-mail servers setup to transparently verify each incoming e-mail against blacklisting databases (a free, uncontrolled and unmanned service) - would lose messages sent from us after an "abuse report" (see my previous message) was processed by a "spam protector" (which is a fully automated procedure) and until the time someone from "accused" network had the issue resolved (this is why it's so important to jump immediately on the issue and spend one's personal time resolving it).
The situation is absolutely ridiculous and unavoidable for most service providers - each day one or another "dumb" spam protector, silently or by sending a message to us, blacklist our servers in their databases. The rest just depends on a blacklisting database used by certain ISP, organization or individual. Believe it or not - many organizations use _multiple_ blacklisting databases - because they are free... their users don't even know that they might have lost certain important message. Sure, it blocks some SPAM too...
Yes, you're apparently right about billing messages being personalized while this one was sent as "canned".
We'll think about what we can do there, in order to generate such a "canned" messages being coupled with some personal information in the body or in "subject" of the message (won't prevent people from reporting them to spam blacklisting DBs, but will provide some more options to receive it by other servers).