Restricting emails to Unsubscribe users

Hi

We want to restrict emails to particular external email ID's. Basically these are users who want to unsubscribe from our emails, both from webmail and mail clients. Can anyone point me in the right direction.

Our users send emails to multiple recipients and we want only the unsubscribed email to be restricted and rest should be delivered.

I am using zimbra 8.6.

Thanks.