News: "Junk Email Filter" is broken and you shouldn't use it

We've seen a small trend where people set their backup MX records to Junk Email Filter, a third party service. According to them, this functions as a backup MX and helps them identify spam email to improve the quality of their product. In reality, they blacklist every IP that sends an email to it. They say this isn't how it works, they tell us we're mistaken, we're not. They are either mistaken or intentionally lying about it. Here's what we've noticed:

You'll send an email to someone who uses Junk Email Filter as their backup MX record. Their primary mail server is down. Our mail server attempts the backup MX, as it is supposed to do, and then Junk Email Filter blacklists every IP we try to send it from. This happens regardless of the email in question, and not one single time have we seen it occur when the email sent was actually spam or even easily mistaken for spam. Personal emails, transactional emails, not marketing, not spam.

So if your recipient uses Junk Email Filter, has their primary MX down, and causes our IP(s) to become blacklisted, we will block that recipient from receiving mail from our customers. If you don't want your email address to be blacklisted, you should absolutely not use Junk Email Filter as a backup MX. They are either incompetent or malicious, and neither is acceptable.