Notification when sending an email that one or more people on the email have unsubscribed and won't receive your email

I have found that parents inadvertently unsubscribe themselves to our emails, and then when I send important emails they never receive them but I have no idea they have unsubscribed. It would be great if when I go to send emails, a box pops up that says 'the following people have unsubscribed and will not receive this email'. Thanks1