We regularly help businesses migrate from one Office 365 to another Office 365 tenant. A verycommon use case involves the transition between (or within) email providers or splitting out business units or the migration of tenants between organizations. Microsoft warns that a new Office 365 tenant may be inaccessible for as long as 24 hours during the migration, which means there is a chance for email to be lost. Microsoft’s guide to migrating email accounts between Office365 tenants includes the following instruction:
Change your primary MX record from Office 365 to domain that is not reachable, i.e. “unreachable.example.com”. Internet mail servers attempting to deliver new mail will queue the mail and attempt redelivery for 24 hours. Using this method, some email may return a non-delivery report (NDR) depending on the server attempting to deliver the email. If this is a problem use an MX record backup service. There are many third party services that will queue your email for days or weeks. Once your migration is complete, these services will deliver the queued mail to your new Office 365 tenant.
If non-delivered email is unacceptable, our backup MX service can be employed to temporarily queue all emails during the tenant transitions – we can queue your email while your tenant is migrated with geographic redundancy, technical support and expert guidance for $200 for the migration. Take a look at https://www.tenantmigration.com/ for additional information