Hi Team, We are working on device migration from one domain to another across different forests, using the ForensiT tool. We have 95% of the users' mailboxes in the M365. After migrating devices to the new domain, we observed that the old .ost files still exist. When we log into Outlook with a new domain user account, a new connection is established, and new .ost file is created. This results in two .ost files for the same user: one from the old domain (abc.com) and one from the new domain (xyz.com). We are concerned about what should be done with the old .ost files (abc.com) before or after migration. Can we point the old .ost file (abc.com) to the new .ost file (xyz.com) after the migration? If so, how should we proceed with this? If there are any other alternatives, please share what can be done with the old .ost files before or after migration. We need your best recommendation and best practices for .ost file that reside in device before migration and after migration creates a new .ost file when login outlook with new domain account. Thank you!

Hi @ Prabhjot Singh , Welcome to our forum! According to your description, i notice that you use ForensiT tool to migrate. Please kindly understand that the Outlook tag here we mainly focus on general issues about Outlook desktop client and known little about ForensiT tool. Regarding the old and new .ost file as you mentioned, please log in to OWA to check if there're any changes between before and after migration. If there's no changes, when you configure your account as Exchange in Outlook, new .ost file will be created and the old .ost file has no effect, you can remove it. Hope to help you! If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment". Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.

Migrating Devices Across Domains: Handling Old and New .ost Files

Accepted answer

ChristyZhang-MSFT 23,726 Reputation points Microsoft Vendor

2024-06-26T07:45:12.7566667+00:00

Hi @Prabhjot Singh ,

Welcome to our forum!

According to your description, i notice that you use ForensiT tool to migrate. Please kindly understand that the Outlook tag here we mainly focus on general issues about Outlook desktop client and known little about ForensiT tool.

Regarding the old and new .ost file as you mentioned, please log in to OWA to check if there're any changes between before and after migration. If there's no changes, when you configure your account as Exchange in Outlook, new .ost file will be created and the old .ost file has no effect, you can remove it.

Hope to help you!

If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".

Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.
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Prabhjot Singh 220 Reputation points

2024-06-26T08:01:19.19+00:00

@ChristyZhang-MSFT , Can we link the old .ost file (abc.com) to the new .ost file (xyz.com) after migration? If so, how should we proceed? We would like to continue using the old .ost file. Is there a way to use the old .ost file with the new one, or merge the old .ost file with the new .ost file?

I mentioned ForensiT because it will migrate the device along with all user data. Before migration, we are removing the old domain Outlook profile for the user. After migrating the device to the new domain using ForensiT, all configurations and settings remain intact. After logging into the device with new credentials, the old .ost file exists. When we log into Outlook with the new credentials, a new profile and a new .ost file are created.

My concern is whether we can merge or link the old .ost file to the new .ost file, considering the large size of the .ost file. Is there a way to use the old .ost file in the new domain? We want to use .ost file.

ChristyZhang-MSFT 23,726 Reputation points Microsoft Vendor

2024-06-26T08:19:02.82+00:00

Hi @Prabhjot Singh ,

Thanks for your reply!

Can we link the old .ost file (abc.com) to the new .ost file (xyz.com) after migration? If so, how should we proceed? We would like to continue using the old .ost file. Is there a way to use the old .ost file with the new one, or merge the old .ost file with the new .ost file?

After migrating the device to the new domain using ForensiT, all configurations and settings remain intact. After logging into the device with new credentials, the old .ost file exists. When we log into Outlook with the new credentials, a new profile and a new .ost file are created.

No., you cannot. As you mentioned, new Outlook profile and new .ost file is created when you add you account in Outlook with the new credentials. It's an expected behavior. If you are concerned about the file size, you could remove the old .ost file, it will not affect the current new .ost and Outlook profile.

Prabhjot Singh 220 Reputation points

2024-06-26T08:27:04.2066667+00:00

@ChristyZhang-MSFT Additionally, data migration from the old domain tenant to the new domain tenant will be completed before or concurrently with the device migration. Therefore, can the .ost files be removed before the device migration? Am I correct in this understanding?

ChristyZhang-MSFT 23,726 Reputation points Microsoft Vendor

2024-06-26T08:35:42.55+00:00

Hi @Prabhjot Singh ,

Thanks for your reply!

Therefore, can the .ost files be removed before the device migration?

It depends on the migration by ForensiT tool. As i mentioned above, we know little about it.

After migrating the device to the new domain using ForensiT, all configurations and settings remain intact.

You can contact support of ForensiT tool to confirm if all data will remain after migration. If you confirm all data will remain after migration by ForensiT tool, you can remove the old .ost file before migration.

Hope to help you!

Prabhjot Singh 220 Reputation points

2024-06-26T08:38:40.0566667+00:00

We have confirmed and also check from our end using ForensiT tool for device migration all data will remain after migration.

ChristyZhang-MSFT 23,726 Reputation points Microsoft Vendor

2024-06-26T08:43:06.64+00:00

Hi @Prabhjot Singh ,

Thanks for your reply!

In this case, you can delete the .ost file before migrating.

By the way, if you think my reply is helpful, would you mind marking the helpful post as an answer? Which would benefit others who also has similar issues in forum.

Prabhjot Singh 220 Reputation points

2024-06-27T04:26:00.7933333+00:00

@ChristyZhang-MSFT So, we can delete the old .ost and .nst files for the previous Outlook profile before device migration.

Will removing these .ost and .nst files cause any issues for the user before device migration?

Additionally, we cannot link or merge the .ost file from the old profile to the new one.

Even if the data migration is not completed, we can still delete the .ost and .nst files from the old Outlook profile before the device migration to new domain.

Also, is there a PowerShell script for deleting those files .ost and .nst from device.

Prabhjot Singh 220 Reputation points

2024-06-27T04:56:26.9866667+00:00

@ChristyZhang-MSFT ?

ChristyZhang-MSFT 23,726 Reputation points Microsoft Vendor

2024-06-27T05:35:39.2133333+00:00

Hi @Prabhjot Singh ,

Will removing these .ost and .nst files cause any issues for the user before device migration?

As i mentioned above, if you confirm all data will remain after migration by ForensiT tool, you can remove the old .ost file before migration and then add your account as Exchange type in Outlook.

Also, is there a PowerShell script for deleting those files .ost and .nst from device.

Please kindly understand that the Outlook tag here we mainly focus on general issues about Outlook desktop client and known little about PowerShell script. Based on my research, i found a similar thread and you can refer to the PowerShell script mentioned by Mike: Powershell script to delete OST file for all users.

(Note: Since the web site is not hosted by Microsoft, the link may change without notice. Microsoft does not guarantee the accuracy of this information.)

Prabhjot Singh 220 Reputation points

2024-06-27T05:39:06.7666667+00:00

@ChristyZhang-MSFT Even if the data migration is not completed, we can still delete the .ost and .nst files from the old Outlook profile before the device migration to new domain. Will removing these .ost and .nst files cause any issues for the user before device migration?

ChristyZhang-MSFT 23,726 Reputation points Microsoft Vendor

2024-06-27T05:51:05.9733333+00:00

Hi @Prabhjot Singh ,

No. Because the data is saved on the server, when you re-add the account, new .ost will be created.

By the way, if you think my reply is helpful, would you mind marking the helpful post as an answer? Which would benefit others who also has similar issues in forum.

Prabhjot Singh 220 Reputation points

2024-06-27T06:06:35.1666667+00:00

@ChristyZhang-MSFT , Thanks for the reply, I also have another in my mind, how can we implement bandwidth constraints for 12,000 devices when new .ost files are created and begin downloading emails?

ChristyZhang-MSFT 23,726 Reputation points Microsoft Vendor

2024-06-27T06:26:51.3533333+00:00

Hi @Prabhjot Singh ,

Glad to help you! If you think my reply is helpful, please remember to mark it as an answer.

On the other hand, please kindly understand that the Outlook tag here we mainly focus on general issues about Outlook desktop client and one thread only discusses one issue. If you need more help about bandwidth, please post your issue as a new thread and add network-related tag for more professional suggestions.

Thanks for your understanding!

SokiGuo-MSFT 28,146 Reputation points Microsoft Vendor

2024-06-28T07:40:51.99+00:00

Hi @Prabhjot Singh ,

I am writing to see how everything is going on with this thread.

Do you need more help with the problem of deleting .ost files? If not, could you please mark "Accept Answer" on the appropriate answer so other users can reference it?And, if you have any further questions do let us know.

Prabhjot Singh 220 Reputation points

2024-06-28T19:02:05.1066667+00:00

Hi, there two different domains across forest. Just wanted to ask a question, like we want to use existing old ost file abc.com within new .ost file in xyz.com. Pointing the abc.com .ost file to xyz.com .ost file will help to do so. If yes, could you provide the way to proceed.Or, Is there a way to use old .ost file with new outlook profile created in xyz.com domain outlook by deleting the new .ost file. Because we have ost files sizes more than 10gb.

ChristyZhang-MSFT 23,726 Reputation points Microsoft Vendor

2024-07-01T05:55:54.9+00:00

Hi @Prabhjot Singh ,

Hi, there two different domains across forest. Just wanted to ask a question, like we want to use existing old ost file abc.com within new .ost file in xyz.com.

No, .ost files for two separate accounts cannot be used together. If you want to access the data of the abc.com in xyz.com account, it is recommended that you could export the data of the abc.com account to a .pst file, and then import the .pst data to the xyz.com account.

More information:

Export emails, contacts, and calendar items to Outlook using a .pst file

Import email, contacts, and calendar from an Outlook .pst file

Prabhjot Singh 220 Reputation points

2024-07-01T15:36:04.4666667+00:00

Hi @ChristyZhang-MSFT , So, , it is recommended that you could export the data of the abc.com account to a .pst file, and then import the .pst data to the xyz.com account. What is the limit size of .pst file? Like we have more than 10 GB of data. It can also exceeds to 20 GB up to also. Is .pst file able to handle more than 10 GB data and import.

Prabhjot Singh 220 Reputation points

2024-07-04T06:27:49.08+00:00

@ChristyZhang-MSFT , I just wanted to ask a question: can we rename the old .ost file from user@abc.com to user@xyz.com.ost and replace the new .ost file with the old .ost file renamed for the new user? Just need confirmation, if renaming and replacing\deleting of new .ost file with old .ost file .
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