Mass email not working

This morning I tried to send a mailing to all my customers.
Although I made the selection to send to all customers only 36 mails (out of +1500) are send.
No errors where show, just the mailing succesfully finished.
None of the customers have checked the box to opt out of the mailings, nor can they edit it themselves.

I hope someone nows how to solve this???

Thanks!

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Solved, apparently selecting both available languaes was not sufficient, I selected 'Default' and that solved it...