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Microsoft 365 Transport Rules

Due to a limitation with the Graph API, when migrating calendar data into Microsoft 365, you should enable Exchange Online transport (mail flow) rules before beginning the migration unless you are using the /D207 Custom option in Transend Migration Console. Transport rules prevent Microsoft 365 from sending calendar invitation, update, and cancellation notifications as calendar items are migrated into user mailboxes. Without the appropriate transport rules in place, migration activity can trigger a large volume of unintended calendar notifications — often referred to as a “calendar invite storm” to meeting attendees.

Enabling the required transport rules suppresses these migration generated notifications while allowing calendar data to be migrated accurately without disrupting end users.

Notes:

Transport Rules are not required when migrating from a Microsoft 365 source to a non-Microsoft target (i.e. Google Workspace)
Transport Rules are not required when migrating via the Exchange Web Services (EWS) API
As an alternative to enabling Exchange Online transport rules, you can enable the /D207 Custom option in Transend Migration Console. This option removes attendees from the calendar event header and adds them to the event body, preventing Microsoft 365 from generating and sending unwanted calendar notifications during migration (commonly referred to as a “calendar invite storm).”

Manually Enable Transport Rules in Microsoft 365

Visit the Microsoft Exchange Admin Center and log in with your admin credentials
Navigate to Mail Flow > Rules
Click Add a rule
Give the rule a meaningful name (i.e “Block Outbound Invites Rule”)
Apply the rule if the recipient is an external user
Make sure that the message properties includes the message type Calendaring
Block the message and delete without notifying
The configuration should look like the following image:
For Set Rule Settings, make sure to Enforce, add a severity of High, and Stop processing more rules
Review and Finish
Make sure the new rule has a Priority of 0 to ensure it is enforced. Otherwise select it and Move Up
Enable the rule
Wait about 30 minutes for propagation to take effect on the whole organization
Complete!
Important! When your migration is complete, you must disable Transport Rules.
It is critical you disable the rule after your migration is complete. If you don’t disable the rule, external meeting invites will not be sent out.

Programmatically Enable Transport Rules in Microsoft 365

If you are comfortable running PowerShell scripts, you can use the m365_initialize_graph.ps1 script to programmatically configure the required Exchange Online transport (mail flow) rules before migrating calendar data through Microsoft Graph.

The script creates temporary Exchange Online mail flow rules for calendar messages generated by members of the migration group, helping prevent unwanted calendar notifications from being sent during migration.

The script is available through TMCTools:

Disable Transport Rules After Migration

After the calendar migration is complete, use the m365_restore_graph.ps1 script to remove the temporary Exchange Online mail flow rules created by m365_initialize_graph.ps1.

The script safely restores normal calendar-message handling by removing only the rules created by the initialization script, without affecting similarly named rules that may already exist in your Microsoft 365 environment.