The audience for email messages won't be the same when targeting the same users with a chat or a post message or filtering them in Contacts. With email messages, we automatically exclude users:

1. That don't have an email
2. That have unsubscribed from you
3. That have marked your emails as spam
4. Who bounced any emails you've sent to them

You can view these excluded users in Contacts by using the same filters in your message, as well as Email has any value and Unsubscribed from Emails is true or Marked email as spam is true or Has hard bounced is true 🙌