One of my active members found recently that his profile was "opted out of receiving email" when he didn't get an email blast that was sent out. He insisted that he never unsubscribed, and his profile didn't have the status update "unsubscribed email" that I usually see when somebody hits "Unsubscribe". What happened?
On a profile's activity stream, when you see the activity "So and so was opted out of receiving email" that indicates that they were manually opted out through an action. Head to edit and there should be a checkbox for "Receive emails." To opt them back in, you can check that box and save.
If you see the notation that "So and so unsubscribed" that means that they actively clicked the unsubscribe link in an email blast. If you were to go to that same "Receive emails" checkbox, you would not be able to opt them back in. Instead, that email address would need to take an action on your site making sure to re-confirm they want to receive future emails by clicking "Send me email updates."
With that in mind, you may want to create a signup type page specifically for folks that have unsubscribed in the past, but now want to hear from you again. You may also be interested in learning more about the unsubscribe page which uses tags for different email groups. This way there isn't a binary opt in or out of all email communication.
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