hello,
we recently migrated a customer from a hosted email platform to Office 365 email solution.
let's call:
when we started, their domain name @mathieu.company .com was pointing to Oldplatform.
They already have a Microsoft 365 Tenancy for desktop app licensing with UPN users@mathieu.company .onmicrosoft.com
day 1: To prepare the migration from oldplatform to O365, we added a DNS entry to the O365 tenancy. This way, we were able to change their UPN from @mathieu.company .onmicrosoft.com to @mathieu.company .com.
day 2: The next morning, all users had to re-authenticate for their Office desktop app, and that was fine.
day 3: in the morning, all emails users were sending from their Outlook desktop App never reached oldplatform, they stayed in the O365 Tenancy. External emails were not flowing to the Outlook Desktop App. Our workaround at this point was to have all users to use the oldplatform webmail. User on mobile devices did not notice any change.
Some technical people looked at the problem when it happened (and not before it happens, I know), and they told me this is an autodiscover problem.
We have another customer coming for migration with the same profile, and I would like to figure out how to change UPN few days before the email migration happens to configure everything in their tenancy (shared mailboxes, custom transport rules, Distribution Lists, aliases, resources mailboxes...)
Does anybody have a way to avoid the described impact?
thank you