What is a good mitigation strategy for reversal of an office 365 cutover migration gone wrong ?

Im in the process of putting together a project plan for a migration of a 50 person office to exchange online .
The big risk I have identified is that we begin to change the workflow over and for some unforeseen reason mail is not routed to either the exchange online or the existing server .
Is there a point in the process that the mail is routed to both locations so we dont face that risk ?
I am assuming that everything goes fine but it is my responsibility to assume the worse and plan for it .
What are the areas that could affect the mail flow and how would i mitigate during the 72 hour time period we are taking this move ?
Andre PAsked:
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Belal KhalladSR ConsultantCommented:
Hello there,

In a cutover migration incoming mail will always go to your exchange on premise. This is due to the fact that your MX record is pointing to your Exchange On-premise environment. The only time mail will flow to office365 is once you change the MX Record to point to office365.  Check to make sure your smtp domain is not blacklisted, and check to make sure your filrewall is accepting email on port 25. Sometimes the issue could at your ISP, need to check with to ensure they are not blocking port 25.
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Marshal HubsEmail ConsultantCommented:
Please check this link for all information about cutover migration.
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Ajit SinghCommented:
Please have a look at below earlier threads might helps you to get in more detailed:

Office 365 Cutover Migration - Logistics and Post-Migration Strategy

Migration strategy for Office 365 (cutover method)


Hope this helps!
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