Newsletter – subscribers claim not receiving

Hi, sorry. I am sure this has been up several times but cannot find on the forum.
Using newsletter, doing very well.
But I – unfortunately have some core-users that calm they do not receive the newsletter. Not in the mailbox or in the spam filter.
They are all subscribers – triple checked. 217 of them.
In the overview it tells me reach is 217 and open by 50 %.
So, I am happy.
But what can I do in regard to the complainers.