How to Migrate Mailboxes over 100GB to O365?

Summary: As Office 365 subscriptions come with mailbox size limit, it is a difficult task to migrate mailboxes which are over 100GB to Office 365. In this post, we’ve discussed some ways to help you migrate mailboxes over 100GB to Office 365. Also, we’ve mentioned an EDB converter tool that you can use to directly export mailboxes to Office 365.

How to Migrate Mailboxes over 100GB to O365?

On-premises Exchange Server 2016 or 2019 will not impose any limit on the size of the mailbox. However, you need to increase the resources in the storage, backup, and performance of the server. The limit can be imposed manually by the business. Still, most clients don’t impose this limit and end up having large mailboxes. This is not a problem but when it comes to migrating to Office 365, they might end up with some issues.

The first issue is the mailbox size. This depends on the subscription that you will go for. So, before the actual migration, during the planning, one must analyze the data being migrated and consider upgrading some users from one subscription to another.

Before purchasing or committing to an engagement, it is important to first gather the current assets. You will need to generate a report of all mailboxes, shared mailboxes, and public folders which you have in the Exchange Server, including the size and item count of each resource. Then, filter this report to exclude the mailboxes which will not be migrated.

The next step is to filter the small mailboxes and the mailboxes over 50GB. This will help in choosing the Office 365 subscription. The size limit of a mailbox in Office 365 Business Standard subscription is of 50GB. So, this will not work for your mailboxes which are over 100GB. You can setup the subscription to use the Online Archive which will give another 50GB, but you will need to enable the online archive and set a retention policy for emails to be shifted to the archive.

Another option is to upgrade the subscription from Office 365 Business Standard to Office 365 Enterprise E3. This will give a bunch of features apart from storage increase. The storage in Office 365 Enterprise E3 is 100GB for the mailbox and unlimited Online Archive, when you enable the Auto Expanding feature. If you have several mailboxes over 100GB, you will need to upgrade their subscription to the Enterprise level. There is no need to upgrade all the mailboxes as you can have a mix and match of subscriptions in one tenant.

How to Migrate Mailboxes over 100GB to Office 365?

There are two options for migrating the mailboxes which are over 100GB.

1. Clean up the Mailbox

You can clean up the mailbox from any unwanted items, like spam, deleted, and other folders, to try to reduce the size of mailbox. This can be done by checking each mailbox for deleted items and other folder sizes or run the PowerShell script (as given below) to get the number of deleted items and ask the users to empty the respective folder.

Get-Mailbox <username> | Get-MailboxStatistics | fl DisplayName, Total*

PowerShell script to get the number of deleted items

If like most users they delete emails but never clean up their deleted items folder, there could be substantial data which could put the mailbox below 100GB. Then, it would be possible to migrate the mailbox to Office 365 Enterprise E3 subscription without any manual work.

2. Split the Data

You can also split the data to reduce the mailbox size. However, it’s a bit messy as it requires a lot of administrative effort. This would involve export of all the mailboxes which are over 100GB with a date range. For example, you can export data from 2010 till 2015 and 2015 till 2020 into two separate PST files. This can be done by using the PowerShell command (see the example below), which can be then scripted to loop in the mailboxes.

New-MailboxExportRequest -ContentFilter {(Received -lt ’01/01/2020′) -and (Received -gt ’01/01/2015′} -Mailbox user-a ‘ -Filepath “\\EX2019\PST_Export\user-a.pst”


This will export all the content from the mailbox between January 1, 2015 and January 1, 2020. Now, you can migrate the PST file to the Office 365 mailbox, run the archive process, and export the other PST file.

After the migration, since it will be near the limit, it is suggested to create a policy which would automatically shift emails older than 2 years or a time specified by the business, to the Online Archive.


As you can see, it’s quite a burden to clean up or split the data. This course does not include the archive mailboxes you have in your Exchange Server. This can only be done from an online Exchange Server database. As an alternative, you can use a third-party tool, such as Stellar Converter for EDB. This application can open any Exchange Server database of any version. There is no need for an Exchange Server to open a database. With Stellar Converter for EDB, you can open the EDB files and granularly export from them to PST and other formats. With this application, you can directly import the mailboxes into an Office 365 tenant.

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