This took me a moment to find.
The reason was bad address for one recipient causing
whole digest delivery to be deferred (of kernel-digest)
I removed that bogon address, and now it should work.
> Simon Karpen
> karpes@rpi.edu, slk@acm.rpi.edu, slk@karpes.stu.rpi.edu
/Matti Aarnio <matti.aarnio@tele.fi> <mea@nic.funet.fi>