MigrationWiz migrates content in the source mailbox. It doesn’t touch mail that’s still in the transport pipeline when the MX records change, and during a cutover window some always is. In-transit mail sits in one of three places:

• Queued on a sending server, waiting to deliver to the source tenant MX
• In the source tenant’s mail flow pipeline, being processed by transport rules
• In a source mailbox, unread

MigrationWiz covers only the third.

The first group is the one that catches people. Senders that resolved the old MX before the change, or are still holding a cached record, keep delivering to the source tenant, and queued messages typically retry for 48 to 72 hours. Those messages land in source mailboxes after the cutover pass has finished. Catching them takes another MigrationWiz pass or forwarding to the destination mailbox.

My standard approach is to configure SMTP forwarding from every source mailbox to its destination mailbox for 30 days after switchover. Anything that reaches the source tenant after the MX change is forwarded to the destination automatically. That includes in-transit mail, delayed retries, and messages from senders with the old MX cached.

Is post-cutover SMTP forwarding in your plan for catching in-transit mail?

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