You may be already aware that you can have audit on any Office 365 activities, including Exchange Online mailboxes.
In February 2019, Exchange Online mailbox audit is going to be improved by adding audit for item read and access by mailbox owners, administrators and delegates.
This new implementation is replacing the previous MessageBind action, which will be deprecated when the update will be available on your tenant.
You have nothing to do if you have audit enabled with the default configuration, if you have customized your audit settings you may have to update it to ensure you are getting these new audit actions.
If you need more information regarding the Exchange Online mailbox audit configuration you can go there https://docs.microsoft.com/en-us/office365/securitycompliance/enable-mailbox-auditing