According to the simple header displayed in my email program, the message was definitely sent to the owner's address and not to the group address.

Any chance of that address being saved incorrectly?
I mean, some email clients will save email addresses automatically, and usually this cause a lot of troubles.
For example, the person trying to write to the owner's address could be using something like Your Group's name Owner on the Name field, but what matters at the end is the Email field, and this field may well be set to Your Group's address.
Not sure, only a thought :)

Cheers,
AKA Starboy

Sent from a galaxy far, far away.