Topic
When turning on the Exchange Online Feature as show below, K2 requires full access permission to all Mailboxes. This can be a little concerning to provide these elevated rights to a K2 Service Account. The permission is required for Setup, but it is not required after setup. It is recommended to keep this permission in place however.
https://community.nintex.com/t5/Technical-Issues/K2-and-upcoming-changes-to-Exchange-Online-authentication/ta-p/124404
Instructions
Follow the instructions in the KB article above to turn on the Exchange Online feature. Be sure to grant the elevated rights during this setup process, but once the setup completes successfully, the permissions can be revoked. However, if you would like to use the full capability of K2 with Exchange Online, this permission will need to be provided.