Each time an email is sent, the recipient has an option to opt out of receiving any further emails by clicking a link in the footer of the email.
Managing the opt-out list
You can see who has opted out of receiving emails by navigating to Settings → Email opt-outs.
The list will show all email recipients that have opted out, along with a reason they may have given. Not all recipients may have provided a reason.
Restoring a recipient
If a recipient has accidentally opted out of emails, you can restore them by clicking the actions button next to the recipient, and then Restore. You'll be asked to confirm your password, and optionally provide a reason that you're restoring them to the email list.
Make sure that the recipient wants to continue receiving emails before restoring them. You could be violating their Right to Withdraw if you don't have their permission.