SendPress newsletter not received by some

I have a theory why my grumpiest user never receives the newsletter that IS received by about 400 other people:

The Can Spam field.

As I am not in the USA, and none of the users of my website are in the USA, I have never put our address details in that field. HOWEVER, grumpy user and 5 other people have aol.com email accounts – so I wonder if aol.com just doesn’t deliver them.

Yesterday I put the address into the Can Spam field, sent a newsletter, and this time Mr. Grumpy got it.

Can someone from SendPress confirm my suspicion?

Thanks
SP