Well, I'm blind.. Seriously, I am. I an sorry for missing that.
Maybe the mailing servers are not configured to send more than 5 e-mails using multiple recipients. I have not tested your websites for that.
I thought you were using the looping method to send e-mails in bulk. Nevermind, use @ckhawand's solution, as in concept I was suggesting the same, and apparently it is the only one which works.