Below are the ways Contact records get suppressed from a live email send.  

1) Campaign Opt-Out - The tactic belongs to a campaign in which a contact is opt'ed out.

2) Master Suppression - The contact has opt-ed out globally (unsubscribed from all) via email.

3) Email Opt-Out - Depending on implementation, this means the contact has not opt-ed in or has been opt-ed out.

4) Bounce Events - When an email bounce occurs the details are stored in the Events table. Temporary failures are retried every 10 minutes for 24 hours before resulting in a bounced entry logged in the Events table.

5) Duplicate Emails - If this option is set in preferences, duplicate email addresses will be suppressed so the contact only receives the email once. 

6) Spam complaints - When a contact reports a spam complaint to their email provider (such as AOL or gmail), a spam complaint is recorded in the Events table.

Contact with additional questions.