Threat actors are very smart at having various methods of communication. New research by Jigsaw Security shows that threat actors are using specially created email boxes to make it appear as though the accounts emailed are invalid, however the threat actors are using a feature in mail servers to deliver the content anyway.
We are not sure why the threat actors are taking this step but it may be used like a digital dead drop to send messages to perform other actions.
We are already aware of people using the email draft feature to put messages in accounts as sort of a dead drop that never get sent. We have observed this in our forensics work. This new method however we believe it being used for C2 communications to control malware infected devices as well. Customers should look for any outgoing email to mail services not used by your organization, and potentially block access to these resources.
One of the other methods we are starting to see is intelligence organizations using mailboxes that are configured to automatically response with a not a valid user message, but do in fact receive any email sent to the domain. This dead drop like activity is being observed and actively being investigated.
For additional information please open up an RFI (Request for Information) with the SOC.