That's one of the biggest problems with this type of email service.
They are easily used for spam schemes.
This makes normal email use difficult, since many other services will classify emails from this service as spam, even when they are not.
You actually need to provide a small proof-of-work challenge to unblock sending emails with Owlmail. That's not going to stop spammers, just like it does not stop spammers on other email providers, but it makes it a bit more difficulty to run an operation with hundreds of botted emails.
The actual reasoning from spamhaus is typical: "Based on research, analysis of network data, our telemetry, intelligence sources and our experience, it is Spamhaus's considered opinion that AS205775 is operated for the purpose of proliferating bulletproof hosting and cybercrime operations"
Basically: it does not track the heck out of you, so it's probably used for crime. Block it!