Exchange Migration PST Export

Regular Contributor

I have a very bad exchange. The Exchange server barely works. The contents of the mailboxes should be migrated to Office 365.
Because I can't configure anything on the Exchange Server I want to migrate via PST Import and Export. So that the failure of the emails is relatively short I would like to do this in several steps:

1) PST Export
2) Import PST to O365 into the user mailbox
3) Provide users with the mailbox in O365
4) Export PST Delta from the onPremise Exchange
5) Import PST Delta into the O365 mailbox


For this the users should use their onPremise mailbox until step 3. How can I provide the user with 2 temporary mailboxes (one onPremise and one in Exchange online)?
Thanks for your help

9 Replies

Hi @Stefan Kießig


I'd try and see if you can set up a cutover migration if possible, but if the Exchange really is in a bad way and a PST export/import is the only thing it'll respond to, then I'd look at the following.


At point 3, you want to change the MX records to point to your Exchange Online so new email flows there - also change your autodiscover A Record. You will probably need to set up their Outlook profile again, but this will setup Outlook for 365. If they need access to their old mailbox get them to use OWA in the interim until the delta sync has been imported. There will be some delay and possible issues with the PST's do not export out cleanly, but that would be your best bet. I'd still try to see if you can set up a supported Migration scenario if possible. 

Hi @HidMov 


thank you for your answer.

How can I create an online mailbox (step 2) for the users?

In the UI of O365 I get this message


This user's on-premises mailbox hasn't been migrated to Exchange Online. The Exchange Online mailbox will be available after migration is completed


in step 3 I want to change the MX records. But for step 2 I already need the mailbox for the users in Exchange online. The users should use this online mailbox only after step 2 is finished until then they should use the onpremise mailbox.





run below and assign licenses and you should be able to create mailboxes in O365 :-
Set-User -PermanentlyClearPreviousMailboxInfo

also you can set up mailflow connectors between on-prem and O365 and migrate users in batches, below article explains how you can set up a criteria based mail routing:-

Hi @DeepakRandhawa 


thank you for your answer.


Do I have to stop AD Connect and make changes to the user before?

Set-User -PermanentlyClearPreviousMailboxInfo

Output with AD Connect activated

The command completed successfully, but no settings of 'Jon' were changed.

you don't need to stop AD Connect, try this for a user for which you are getting the error :-
This user's on-premises mailbox hasn't been migrated to Exchange Online. The Exchange Online mailbox will be available after migration is completed
and then reassign a license.


I applied it to a user created with ADConnect. But always get the above warning and no change to the user.

If you have AAD connect installed, then you can uncheck the MSExchnageMailboxguid to sycn to office 365.

By doing this it will allow you to provison mailbox for that user and you can then import the data
after going through the article again i realized you need to stop syncing the ExchangeGUID attribute.




I will stop the sync of the attribute via the ADConnect Sync Service and then run the above PowerShe