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.

Don't add an exception directly to the existing Incoming Emails via the Knowbe4 Defend transport rule. Microsoft Exchange allows each exception type to be used only once per rule, and the existing rule already has two header exceptions, so adding another exception of the same type will fail.

Instead, create a new transport rule positioned above the Incoming Emails via Knowbe4 Defend rule. This new rule intercepts messages that have already been redirected before Defend's rule can reprocess them, preserving the necessary MAPI properties.

To create the new rule, follow the steps below:

1. Log in to your Exchange Admin Center.
2. Navigate to Mail flow > Rules.
3. Open the existing Incoming Emails via the Knowbe4 Defend rule and note the CustomerSpecificValue used in its header exceptions. You will need this value when configuring the new rule.
4. Create a new rule and configure it using the values in the table below.

Field	Value
Rule	(e.g., "Defend Calendar Delegation Fix")
Position	Above "Incoming Emails via Knowbe4 Defend"
Apply this rule if...	The recipient is a member of this group – Defend_Users The sender – Is External/Internal – External/NotInOrganization X-MS-Exchange-Inbox-Rules-Loop message header matches @
Do the following...	Set the message header X-Egress-Defend-Sk to the value [CustomerSpecificValue]
Except if...	X-Egress-Defend-Sk message header includes [CustomerSpecificValue]

5. Position the new rule directly above the Incoming Emails via Knowbe4 Defend rule.
6. Click Save.