MailUp automatically unsubscribes recipients when they click on the unsubscribe link in an email sent by the system, in case of feedback loops, and when certain types of bounces occur. This is to maintain excellent list hygiene and high deliverability rates. Here are some of the scenarios in which a recipient is unsubscribed.
Recipients clicking the "spam" button (abuse reports)
Most email clients like Gmail or Yahoo! - although not all of them - provide a "Spam" or "Junk" button. Clicking this button triggers a notice to MailUp, called "Feedback Loop" (FBL). To stop sending messages to these recipients, so to keep a high level of reputation and maximize deliverability, when a recipient takes that account the recipient status is automatically updated to "unsubscribed". It is possible to filter these recipients using the dropdown menu "Unsubscribed from" on the page Recipients > Unsubscribed, choosing "because misuse has been reported" (this text might have changed in your version of MailUp). The report can also be exported.
Recipients clicking the unsubscribe button provided by email clients
Some web email clients have another button beside "Spam" or "Junk", usually called "Unsubscribe".
Unsubscribe option provided by Gmail
This button is linked to an unsubscription link hidden in the message header, i.e. "List-Unsubscribe". Users clicking that link is automatically unsubscribed, regardless of the procedure within the message itself. Unsubscribed users will not receive any messages anymore, except when forced as subscribers again (forced import or manual change of a user status). For further information please see http://www.list-unsubscribe.com/
Recipients with invalid email addresses (hard bounces)
There is a peculiar kind of error called Hard Bounce (usually with error code 550 5.1.1) which refers to non-existing email addresses (e.g. mailbox not found, no such user here…). These addresses are undoubtedly invalid, so they are automatically unsubscribed after returning the very first error and they can be filtered at the page Recipients > Unsubscribed. The quick removal of Hard Bounces from databases is paramount to maintaining a high level of reputation, and to avoiding having messages blocked by antispam systems.