There can be various reasons for your emails not arriving.
However, one of the most common reasons is that the email status of the recipient is NOT confirmed.
Quentn only sends emails to contacts who have completed the double opt-in process (email status). If you still have unknown contacts, you can guide them through the double opt-in process again by starting a campaign with the start element and attaching an email confirmation. Then filter the contacts according to their email status and start the campaign manually (click here to read how to do this).