Microsoft maintains an autodiscover endpoint for all domains they host. Unfortunately, they don't tend to clean up their database when a domain stops using their email service. This can cause some clients, especially Outlook, to continue trying to connect to accounts on M365, even though they are no longer in that system. Here is a knowledgebase article That goes into detail about the situation: Outlook for Windows Always Attempts to Connect to Microsoft 365 (Office).

As for the migration reports, the details of what went wrong should be available in the relevant retrieval log. Since you are migrating from M365, you likely need to review the EWS retrieval log. One of the more common reasons for items to not be migrated has to do with size limits, such as the limits on message and attachment size.

If you want to try migrating a user again, there is the "Delete existing SmarterMail mailbox items" option. This option deletes custom folders and items from the account before beginning the migration. This deletion process is limited to the item types that are being migrated. This can be useful if, for example, you only want to try migrating calendar data again. The user can continue using the other data in their account, while only having the calendar data re-migrated. If this is not a viable option for you, then the redundant email box idea you proposed is a reasonable approach.

Hi Andrew!

In the article "Outlook for Windows Always Attempts to Connect to Microsoft 365 (Office)" there's a precompiled .REG file to download, but it has more entries than those described in the article itself.

Can you please explain those entries?

And possibly update the article properly so that everything is explained correctly?

Hi Brian, is your REG file very different from the one posted in the KB or is it similar?

We use this

Windows Registry Editor Version 5.00

[HKEY_CURRENT_USER\SOFTWARE\Microsoft\Office\16.0\Outlook\AutoDiscover]
"ExcludeExplicitO365Endpoint"=dword:00000001
"ExcludeHttpRedirect"=dword:00000001
"ExcludeHttpsRootDomain"=dword:00000001
"ExcludeScpLookup"=dword:00000001

[HKEY_CURRENT_USER\SOFTWARE\Microsoft\Office\16.0\Outlook\AutoDiscover\RedirectServers]
"smartermail.cloudpros.dk"=hex(0):

Just to keep everyone here updated: Derek has responded to Gabriele's comment on the KB article. Essentially, the original registry file included some local-specific URLs and some of our internal testing domains. The registry file currently included in the KB has been updated to remove our testing domains as well as the locale-specific domains.

Thx Andrew!