I have been working with Outlook Web Add-ins and Compose Message integration for Office Outlook Web Access for a while now. That is, Outlook add-ins that present themselves when the user is composing an email. This scenario is not seemingly as common. But one of my customers has been completely gung-ho about getting all of their COM add-ins converted to the new Office.js model. Some of their solutions will be able to make it there, and some will not. We are finding some roadblocks with mail compose apps.
One area that is currently failing is setting X-headers on an email item. I developed a JavaScript library to assist with this: easyEws. I blogged about it here. The one specific EWS SOAP call my customer wants to use from that library is this one:
updateEwsHeader: Updated the named x-header in the message.
This function works great in two conditions:
- Outlook Web Access (or Office 365 Outlook Online), and
- Outlook client – Online mode only
If you use Outlook client (Outlook 2016) in cached mode, this call will seem to work (because it does), but the x-headers will be missing when the user presses “Send.” The problem is that while the currently composed item is saved to the drafts folder and the custom x-headers are written to it via the EWS call (server side), the current Outlook Inspector/session is NOT updated. The cached Drafts folder may take (depending on network latency) anywhere from 1 to 3 minutes to be updated, but regardless the item in the current inspector session will NOT ever be updated. So when the user presses SEND, the item in the drafts folder will be overwritten with one that does not contain the custom x-headers and then sent.
There is no way around this limitation. And I have confirmed this with the Exchange and Office.js teams that this is a limitation of cached mode in Outlook. If you have a need for custom x-headers with an Outlook add-in your only option for now is to create (or keep your existing) COM/VSTO add-in.
If you are running into this limitation, please let me know by commenting below.