Some email providers notify Groups.io when group messages end up in the provider’s users’ spam folders. Most likely, the group messages were automatically flagged as spam by the email provider rather than having been specifically marked as spam by the users.

When Groups.io receives such a notification from an email provider, the system automatically unsubscribes that member from the group. Groups.io also sends the member an email with a link that lets them automatically and quickly resubscribe to the group if the spam flagging was a mistake.

If you receive a notification from Groups.io that you were removed from a group because of reported spam:

• Click the link in the notification message that automatically resubscribes you to the group.
  Note: The link expires 7 days after the notification was sent.
• Check the spam folder in your email application, and if that folder contains messages from Groups.io, mark those messages as not spam. Doing so will help train the email provider’s spam filters to make fewer mistakes about Groups.io emails.
  ! Important: Do not delete the messages from the spam folder or allow them to be automatically deleted from it. Otherwise, the email provider will likely continue flagging Groups.io messages as spam.

For additional information about this situation, see the wiki in the GroupManagersForum group.