In your account, under the Audience drop-down menu on the left side there is a section titled 'Suppression' - but what is that for?
Suppression is what happens when an email address or phone number can't be reached, for various reasons. These email addresses and phone numbers are then removed from future campaigns, but you can still review who is on these lists to try and get correct / alternate contact information
Here is a breakdown of the items under 'Suppression' and what they mean.
Email Unsubscribes: End-users who received your email(s) and clicked on the 'unsubscribe' button at the bottom or top of the email.
Email Bounces: Unreachable email addresses. Can happen for various reasons, but most popular would be misspellings, expired accounts, or full inboxes.
Email Complaints: Someone who has reported your emails as spam. This one is particularly important, as it can effect your email reputation score.
Email In-active: The recipient's email account is no longer active.
SMS Unsubscribes: End-users who received your SMS message(s) and replied 'STOP' to unsubscribe.
SMS Bounces: Unreachable phone numbers. Main reason would be disconnected line or that the original phone number was input incorrectly.