Not able to send email from Outlook app on macOS

Products

• Cloud Secure Web Gateway - Cloud SWG

Issue/Introduction

Users accessing internet server via Cloud SWG using WSS Agents.

Users running Microsoft Office 365 applications including the new (not classic) Outlook version.

When users compose an email and select SEND, the email appears in the Outbox immediately but sits there indefinitely without actually being sent.

This is seen with all macOS users and a handful of WIndows users.

CASB integrated with Cloud SWG and Office 365 gatelets enabled.

Cloud Proxy access logs report 400 errors for a number of Outlook endpoints at the time the issue was seen.

Environment

CLoud SWG.

CASB.

Office 365 Applications.

WSS Agents.

macOS.

Cause

The outlookservice streams (https://outlook.office365.com/outlookservice and https://outlook.office.com/outlookservice) do not want to be touched, and the CASB ICAP connection broke the stream.

"outlookservice.hxs" is a URL path associated with the new Outlook application on Windows and macOS. It's related to the communication between the Outlook client and the Outlook.com or Office 365 backend. Specifically, it's used in the servicechannel.hxs endpoint for email sending, receiving, and mailbox synchronization. 

Resolution

Contact Broadcom support team to apply the change via a policy fragment. This change will be rolled out permanently to all tenant within next few months at a maximum.

Additional Information

Access logs for the above outlookservice.hxs endpoints constantly show the 400 HTTP status response with invalid_request verdict when intercepted.

Issues does not appear when CASB gatelet for Office 365 disabled.