If your emails do not arrive, it can have different causes.
However, one of the most common is that the recipient's email status is NOT Confirmed.
Quentn only sends to the contacts who have completed the double opt-in (see email status).
If you still have unknown contacts, you can put them through the double opt-in again by starting a campaign with the Start element and attaching an email confirmation. Then filter the contacts according to the email status and start the campaign manually (you can find out how to do this here).