Can Outlook custom properties in an email be viewed by another user

Question

Hi all,

I am developing an add-in that saves custom properties on an e-mail using the "saveAsync" method of the Office-js api: https://learn.microsoft.com/en-us/javascript/api/outlook/office.customproperties?view=outlook-js-preview#outlook-office-customproperties-saveasync-member(1). I am able to view the properties if I create the e-mail, save the properties using the add-in and send it to myself. Now, if I send that same email to another user, that user is not able to pull the properties that were saved by me on the email. I am using EWS request to fetch the data and I am not seeing the properties as the other user https://learn.microsoft.com/en-us/office/dev/add-ins/outlook/metadata-for-an-outlook-add-in#custom-data-per-item-in-a-mailbox-custom-properties.

Are custom properties only retrievable by the user who saved them?

Answer 1

Custom Properties are not transmitted.
This means that they are stripped from the outgoing email that eventually arrives in the Inbox of your recipients, including you.
However, these properties are available on the item in your Sent Items folder.

If you want to stamp persistent information on an item that will be available to recipients, consider using our Internet Headers API.