Using the Tag for external email messages received feature in the Microsoft Graph and Exchange Web Services
The "Tag for external email messages received" feature was introduced into Office365 recently to help people better to identify mail that comes from external sender vs internal sender see this for more info on this feature . What happens when you enable this feature is that for messages with External sender a new Mapi property is set on those messages eg For messages from internal senders the property doesn't appear to get set and if the feature isn't enabled in your tenant then you won't see this property either. You can negate the boolean value of the property which will turn off the external tag. Using it in the Microsoft Graph API If you want to use it in the Microsoft Graph API when you retrieve messages you can include this as a SingleValueExtendedProperties eg for Retrieving and filtering message you can use I've included this in my Get LastEmail 101 graph sample if you want to try this in a real mailbox eg you can do Get-LastEmail -InternalSender -M...