This feature is currently in Walk, and supported in databases created after September 20, 2021.
DISCO now allows searching for email communications with a unique count of domains through syntax domainCount. This can be used in conjunction with the existing domain field to easily find privileged communications between a law firm and client.
DISCO also allows users to search communications by participant, recipientCount, and participantCount. Participant allows the user to search for a specific participant that is found on an emails From, To, CC, or BCC fields. RecipientCount allows the user to search for communications with a unique count of recipients (To, CC, or BCC fields). ParticipantCount allows the user to search for communications with a unique count of participants.
More information and search syntax examples can be found in Searching emails for privileged communications and communication participants.