Contacts can select the unsubscribe link in your email campaigns to unsubscribe from a list at any time.
You can view all unsubscribe actions for a campaign in the unsubscribe report.
This also includes contacts who reached their bounce threshold (as set under Contacts > Suppressed contacts > Bounce settings), and those who've used the option to report your email as spam/junk in their email client.
These addresses are automatically removed from your lists so that they can't be emailed in future. This would apply even if these email addresses were included in a future upload to the same or different lists. They'd be automatically excluded from the upload.
A complete list of all addresses removed can be found under Contacts > Suppressed contacts.
View the unsubscribes report for an email campaign
To view your unsubscribed report:
Go to Analytics > Reports.
Select the campaign you want to view the report for.
On the Email report, expand the MORE REPORTS drop-down menu and select Unsubscribes.
Using the Unsubscribes report
This report shows all unsubscribed contacts.
Any subsequently deleted contacts in the report are shown as Deleted contact (apart from deleted suppressed contacts whose email addresses will need to be retained).