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:


REST: REST API only

EWS: EWS only
GRAPH: Graph API only
EWS/ REST: EWS and REST API
All: EWS + Rest API + Graph API


Windows

Windows 

Clients Windows Outlook 

MS 3651 

Outlook 2019 

Outlook 2016 

Outlook 2013 

Server

Exchange Online 

All

All

EWS 

EWS  

Exchange 2019² 

All

EWS/ REST

EWS 

EWS  

Exchange 2016² 

EWS/ REST

EWS/ REST

EWS 

EWS  

 

 

macOS

macOS 

Clients macOS Outlook  

MS 3651 

Outlook 2019 

Outlook 2016 

Server

Exchange Online 

All

EWS/ REST

EWS/ REST

Exchange 2019² 

EWS/ REST 

EWS/ REST

EWS/ REST

Exchange 2016² 

EWS/ REST

EWS/ REST

EWS/ REST

 

 

Other:

 

Clients Outlook 

Android App 

iOS App 

Desktop Browser

 

Mobile Browser

Server

Exchange Online 

REST 

REST 

EWS/ REST

not supported

Exchange 2019² 

REST 

REST 

EWS/ REST

not supported

Exchange 2016² 

REST 

REST 

EWS/ REST

not supported

1 Microsoft Microsoft 365 subscription

2 connected to Exchange Online (hybrid deployment)


Differences in forwarding via EWS and REST/Graph 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:

 


via Rest / Graph via EWS
.eml mode
  • mail.eml
  • mail.eml
  • for emails greater than 500kB the add-in automatically switches to split mode
Split mode
  • body.html
  • header.text
  •  All attachments as the original files³
  • body.html
  • headers.txt
  • attachments.txt³
  •  contains information about the attachment`s name, size, type, isInline


3 If the email contains attachments