Meanwhile I rolled back to v0.15a with:
cd mailinabox
git checkout v0.15a
setup/start.sh
My status check emails are working fine again. Here’s an output:
mailbox.technofaq.org – Previously:
Your box’s reverse DNS is currently radio.samizdatsounds.org, but
it should be mailbox.technofaq.org. Your ISP or cloud provider
will have instructions on setting up reverse DNS for your box.
mailbox.technofaq.org – Currently:
✓ Reverse DNS is set correctly at ISP. [198.91.88.112 ↦
mailbox.technofaq.org]