When a calendar invite is sent to an executive who has a delegate, such as an executive assistant, the invitation may fail to process correctly in the delegate’s workflow.

The common issues associated with this are as follows:

• The calendar invite remains marked as Tentative on the executive’s calendar even after the delegate has accepted the invitation.
• The meeting is accepted on the delegate’s calendar instead of the executive’s.
• RSVP responses from the delegate are lost or not correctly associated with the executive’s mailbox.

Root Cause

This behavior occurs due to how Microsoft 365 handles inbox rules and MIME transitions. When an invite is redirected from an executive to a delegate, it can be processed by Defend for a second time. During this second pass, specific TNEF or MAPI properties, such as “searchkey”, are stripped. Without these properties, Microsoft Outlook cannot identify the “on behalf of” relationship, so it treats the delegate as the primary attendee.

Resolution

To resolve this issue, you must prevent Defend from reprocessing the emails that have already been redirected by internal inbox rules. This process is achieved by adding an exception to an existing Defend transport rule.

To add the required exception to your Defend transport rule, follow the steps below:

1. Log in to your Exchange Admin Center.
2. Navigate to Mail flow > Rules.
3. Locate and edit the rule named Incoming emails via Defend.
4. Navigate to the Except if… section and select Add exception.
5. Using the drop-down menu, select The message headers… > matches these text patterns.
6. Enter the following details.
   1. Header name: X-MS-Exchange-Inbox-Rules-Loop
   2. Matches: @
7. Click Save.