Outlook Classic not connecting to new Microsoft 365 Tenant

[calapso]

We've moved from a shared Microsoft 365 environment to our own separate tenant.

 

As part of this, we created PST files (email backups) for all users from the old shared tenant using Outlook Classic.

To continue using our existing email domain in the new tenant, we had to delete it from the old shared tenant first. Once deleted, we added it to our new Microsoft 365 A1 tenant and updated the DNS and name server settings. Everything shows as correctly configured (green ticks).

I then recreated an email account in the new tenant using the same address as before. However, when I try to add this account to Outlook Classic (so I can import the PST file), it fails to connect to the Exchange server.

To test further, I created a different test email accounts on the same domain. ie test.user@domain name etc .These connected to Outlook Classic without any issues.

So the domain setup seems fine, but for some reason the original accounts won’t connect. I urgently need help figuring out why this is happening and how to fix it. 

Anyone came across this or any suggestion to fix please?

 

Thank you!

[pete]

Does OWA work for the recreated accounts?  As in - mailbox is otherwise functional, but Outlook classic won't connect?

 

Was the Outlook Classic install aware of the previous shared tenant (GPO, other settings) or is this a clean device that's never connected to the previous shared tenant?

[Roberto]

Try using the diagnostics Microsoft have published to see what happens with a broken vs. working mailbox. 

 

Microsoft Remote Connectivity Analyzer

[calapso]

On 28/05/2025 at 09:30, pete said:

 

 

On 28/05/2025 at 09:30, pete said:

Does OWA work for the recreated accounts?  As in - mailbox is otherwise functional, but Outlook classic won't connect?

 

Was the Outlook Classic install aware of the previous shared tenant (GPO, other settings) or is this a clean device that's never connected to the previous shared tenant?

Hi Pete,

 

Thank you for responding.

 

Yes, OWA does work for recreated accounts but will not connect on Outlook Classic. 

The Outlook Classic was aware of the previous shared tenant. 

 

Also noticed on one of the user's PC that when she tried to log onto the account on the new tenant it says password incorrect. This Outlook classic that was talking to the previous shared tenant and looks like it is trying to connect to the old one. 

I have tried clearing out the cache in outlook and also delet the discover files etc but no joy.

Looks like Outlook stores some data elsewhere that it is fetching from but dont know where. 

 

Edited June 18, 2025 by calapso

[mavhc]

Check appdata, failing that, registry