Exchange 2013 and Office 365 hybrid environment. One O365 user sent a message to 5 people, 2 people totally outside of the organization (Gmail) and 3 people were CCd inside the organization. 1 of the internal people that was CCd keeps getting the message every single minute. Only the original is in the users sent items (which i since deleted per other tech articles) and none of the other recipients received more than the first original email.
I have checked the senders Outbox and sent items on both office PC with outlook 365 and on the portal and the original message is no longer there.