Malware
LunarMail
According to ESET Research, this is a Outlook Add-In that can use email messages for its C&C communication.
LunarMail is a Windows malware family operated by Turla.
Background
ESET Research reports that this is an Outlook Add-In capable of relaying its C&C communication through email messages.
Source: Malpedia (Fraunhofer FKIE).