Limits for migrating O365 mailboxes

Is there any limitations for migrating mailboxes from one O365 tenant to another. I am in a situation where there are very big mailboxes of 200+ GB in size and how many days might needed to migrate these mailboxes?

  • Hello  ,

    Standard Microsoft 365 mailboxes are limited at 100GB in size. Are you referring to Archive mailboxes that are 200GB+ in size? If so, then this KB article describes migrating more than 100GB of data into an auto expanding archive. After migrating the initial 100GB, it will take time for the target archive mailbox to expand and allow for more data to be migrated. You may be able to create a Service Request in the target M365 tenant and request that Microsoft manually expand the larger archive mailboxes.

    For the best migration throughput, you will need to request a relaxed "EWS throttling policy" from Microsoft in both the source and target tenants, as described in this article. However, throttling isn't lifted, just relaxed. 

    Unfortunately, I can't really estimate the amount of time it will take to migrate this data, it depends on what types of messages the mailbox consists of, as well as how long it takes for the mailbox to auto-expand to accept more data. There are just too many variables involved here.

    Please let me know if you have any additional questions or need any additional clarification. 


    Trevor Taegder
    Senior Technical Support Engineer
    Quest | Support

  • Yes these are data from archive mailbox. If I remember there was a limit from quest on demand that migrated only 10gb per day. I would like to know if that limit is still applicable while migration 

  • Hi Vinay, Quest doesn't apply a limit on the volume of content you migrate in a day. I think you might be referring to our suggestion when using ODME to migrate Gmail, where we recommend setting the limit to prevent the admin and user account from being locked out by Google when trying to migrate more than 10GB per mailbox.   

  • Yes thats right I have seen this limit for Google to M365 migration. I would like to know if there are any size limitations for M365 tenant to another M365 tenant migration. I would be migrating all the data from Primary Mailbox, Archive mailbox and One Drive.

  • We have no limit on data in a OneDrve or Mailbox migration. I would only say that you shouldn't take all versions of a file on OneDrive, since that will really slow you down.