Thanks for the idea! I've added a test for PTR:
https://github.com/JoshData/mailinabox/commit/ec11241a7a619a...
https://github.com/JoshData/mailinabox/commit/ec11241a7a619a...
* Test/alert if your PTR mismatches your HELO name
* If your IP is on any of the major RBLs
* Point admins toward AOL/Yahoo/ReturnPath whitelists and feedback loops.