Is there a Pinjo for Unix/Linux?

No there isn’t. But if you still want to try it you can run Pinjo on another (windows based) machine.

And if you still doubt the stability of Windows, or want to build in some extra safety, modify your DNS MX records for safety like below:

mailserver.com.   MX   5  pinjoserver.mailserver.com
mailserver.com.   MX  10  mail.mailserver.com

where pinjoserver.mailserver.com is your server with Pinjo running on it and mail.mailserver.com is your original mailserver. If your windows server for any reason should fail, your original mailserver takes over as if nothing happened. A benefit of this method is that you don't have to change anything to your original mailserver!