The value we are trying to get is the Message ID, which corresponds to the internetMessageId property of the email response body when using Graph API:

"internetMessageId": "DN5DD04JB32972053F856D95A0276GF497F271@DN5DD04JB3297.test.prod.outlook.com"

I noticed there's an EmID reserved OPM column, but this doesn't seem to be the Message ID.