Change the MX record before the BitTitan jobs finish, and new inbound mail starts landing in the destination mailbox while the user’s older mail is still migrating from the source.

That split is manageable on its own. When MigrationWiz finishes, the historical data arrives in the destination and the two streams sit together without issue. The trouble starts if the migration is delayed and the source mailbox keeps receiving new inbound mail after the MX change. Then the source tenant has to forward that mail to the destination, or routing becomes ambiguous.

To avoid it:

• Update the MX record after the final migration pass has completed for every user in the wave
• Lower the MX record’s DNS TTL to 300 seconds or less, 24 to 48 hours before cutover, to limit propagation delay once you make the change

Is your MX record change timed to your final MigrationWiz pass?

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