Both COM add-ins and Web add-ins don't provide any internet headers until the message is sent out. You may expect the internet headers set when the message is put to the Sent Items folder, but before that they are not created and stamped by the service/transport provider.
You can specify data specific to an item in the user's mailbox using the
CustomProperties object. For example, your mail add-in could categorize certain messages and note the category using a custom property
messageCategory. Or, if your mail add-in creates appointments from meeting suggestions in a message, you can use a custom property to track each of these appointments. This ensures that if the user opens the message again, your mail add-in doesn't offer to create the appointment a second time.
Be aware, changes to custom properties are stored on in-memory copies of the properties for the current Outlook session. To make sure these custom properties will be available in the next session, use
These add-in-specific, item-specific custom properties can only be accessed by using the CustomProperties object. Like I wrote earlier, these properties are different from the custom, MAPI-based
UserProperties in the Outlook object model, and extended properties in Exchange Web Services (EWS). You cannot directly access
CustomProperties by using the Outlook object model, EWS, or REST. To learn how to access
CustomProperties using EWS or REST, see the section Get custom properties using EWS or REST.
You can post or vote for an existing feature request on Tech Community where they are considered when the Office dev team go through the planning process. Use the github label:
Type: product feature request at https://aka.ms/M365dev-suggestions .