An email was sent on behalf of another user that should not have been sent. Multiple people have this permission. How do I find out who sent the email? Assume it's been deleted from sent items. Using Office365 hosted exchange.
If you have audit log enabled (it isn't by default) in the security and compliance centre it tells you this information.
You will want to run a message trace from the Office 365 administration center. Note that there are different instructions for emails that are either under or over 7 days old.