This is a BUG (or maybe a feature requested by Microsoft?) in Outlook - if you register your domain on Microsoft 365, Outlook Autodiscover for MAPI ALWAYS tries to connect to Microsoft 365 and won't connect to other services.

You can avoid this with a mod in the Windows registry that tells Outlook NOT to use Microsoft 365 with the Autodiscover service.

At the moment I don't have a tutorial to do it, I will post it later if I can (but you can find it by searching the community ...)

Hey Matteo, sorry to hear you're having trouble with this, and I hope we can save you some time here! I believe you may be able to get those existing profiles open with a couple of modifications to those client PCs beforehand. First, you'll want to implement the registry key change outlined here:

That tells Outlook to prefer the XML it gets back from SmarterMail. Next, you can add an entry like the one below to the client PC's C:\Windows\System32\Drivers\etc\hosts file: 

127.0.0.1        outlook.office365.com

Once you get that added to the file, save it to the desktop (it won't let you save directly to that path) then copy the replacement (with no file extension) in to replace the original. Finally, give the PC a reboot to force the changes into place. That change will tell the PC to redirect calls to outlook.office365.com back to the local results it got from SmarterMail. Can you give this a shot and let us know if it helps?

That's the commands that I usually run

reg add HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\AutoDiscover /t REG_DWORD /v PreferLocalXML /d 1
reg add HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\AutoDiscover /t REG_DWORD /v ExcludeExplicitO365Endpoint /d 1