The Outlook Add-In uses Exchange Web Services (EWS) or the Outlook REST API to retrieve data from the user's Outlook mailbox.

Below you can find the availability of the EWS and REST API for every supported Exchange Server / Outlook client combination and their impact on forwarding.

The REST API is the newer interface and offers more possibilities in comparison to EWS. The limitations of using EWS are listed later in the article.

Exchange On-Premises

For all Exchange On-Premises servers (no hybrid deployment) we can only support EWS.

Exchange Online/Hybrid Server Deployments

For Exchange Online and hybrid deployments of Exchange Servers, the following client/server combinations for EWS and REST API are available:


EWS: EWS only

Both: EWS and REST API




1 Microsoft Microsoft 365 subscription

2 connected to Exchange Online (hybrid deployment)

Differences in forwarding via EWS and REST API

Forwarding can be done in “.eml” or “split” mode. Depending on the available API and the configured forwarding mode, the following files are forwarded to the customer’s email addresses:

3 If the email contains attachments