Whenever a bulk email is sent out through the platform, the receiver has an option to Unsubscribe through a link at the bottom of the email.
Clicking this link will opt out the user from receiving further emails sent to through that emailing group.
Here's how you can add a Unsubscribe link to an email -
As an admin, you can see this list of users who opted out, through Unsubscribed Users feature on admin panel. This tells you which of your alumni/constituents have unsubscribed, and from which mailing group.
If you see a particular email sent from you getting a lot of unsubscribes, revisit it to see what you could refine. You can even reach out to users who unsubscribe. Such direct feedback will enhance your messaging strategy.
NOTE: The link unsubscribes you from the particular group that this email went to. This doesn't opt you out of all further communication.
For e.g., someone could unsubscribe from emails sent to the 'Football' Interest group, but still continue to receive emails sent to the class of '87 that they're a part of.
If someone reaches out to you to opt them out from all communication - you can set this preference for that person from Data Studio.