Microsoft 365 Mailbox capacities and sizes

To better understand the mailbox capacities in Microsoft 365, think of an Exchange Online mailbox as potentially being made up of three distinct components like so:


  • Primary mailbox = Can be synchronised to Outlook on the desktop and into an OST file
  • Archive mailbox = Resides in the cloud
  • Compliance mailbox = Provides extra features like unlimited storage, litigation hold, etc. This too only resides in the cloud

The process by which the Compliance mailbox is provided 1.5TB of storage is by adding 100GB blocks of space as required. Thus you start with 100GB and when you exceed that another 100GB is added and so on. You can read about this in more detail here:

Overview of auto-expanding archive

Now the capabilities and capacities of each of these individual mailboxes is defined in the Exchange Online limits, which currently are:



The configuration for Microsoft 365 Business Basic, Business Standard, Office 365 E1 and Exchange Online Plan 1 stand alone look like:


For all these licenses you get a 50GB primary mailbox and a 50GB cloud only archive.


So a user with Microsoft 365 Business Standard like so:


will have a primary mailbox of capacity 50GB:


and an archive also of 50GB like so:


Thus, the total mailbox capacity across primary and archive combined here will be 100GB for these plans.

A Microsoft 365 Enterprise E3, E5, Office 365 E3, E5 or Exchange Online Plan 2 mailbox looks like:


It has a 100GB primary mailbox and a 1.5TB max capacity archive thanks to the fact that the features of the Compliance mailbox are baked into these plans as shown above. Confirming this in the Exchange Online limits documentation:



This 1.5TB capacity is provisioned by Auto expanding archive as mentioned previously per:


Where confusion is common is when the capacity of Microsoft 365 Business Premium mailboxes is considered.


As you can see from the above diagram, Microsoft 365 Business Premium is a little bit special because it takes a standard Exchange Online Plan 1 as discussed previously and adds something called Exchange Online Archiving. In simple terms, think of Exchange Online Archiving mapping directly to the Compliance mailbox mentioned early on. In essence, it provides an Exchange Online Plan 1 mailbox will features like 1.5TB storage, litigation hold and so on.


Thus, an easier way to think about a Microsoft 365 Business Premium mailbox is as being almost identical to the mailboxes found in Microsoft E3, E5, Office 365 E3, E5 and Exchange Online Plan 2 stand alone. That is except for one important difference. The Microsoft 365 Business Premium mailbox has a primary mailbox limit of 50GB which is just like the other Microsoft 365 Business mailboxes. This means that maximum amount of data that can be accommodated by a Microsoft 365 Business mailbox in a local OST file is 50GB NOT 100GB like what you receive with Enterprise mailboxes.

In summary then:

  • All Business mailboxes (and E1) receive a 50GB primary mailbox + 50 GB cloud archive mailbox = 100GB total storage
  • All Enterprise mailboxes (apart from E1) receive a 100GB primary mailbox + 1.5TB cloud archive mailbox
  • Business Premium mailboxes receive a 50GB primary mailbox + 1.5TB cloud archive mailbox


Microsoft 365 Business Premium receives this 1.5TB mailbox capability thanks to the inclusion of Exchange Online Archiving as shown above.

To get the best performance of any mailbox it is recommended best practice to ensure that capacities don’t get anywhere near what is detailed here. However, if you must, just keep the capacities and limitations for your license in mind.

8 thoughts on “Microsoft 365 Mailbox capacities and sizes

  1. What about shared mailboxes?
    The documentation says shared mailboxes start out at 50GB.
    If I create multiple shared mailboxes (info, support, subscribe) all used by the office administrator, does that mean the admin has in addition to their UserMailbox they have 200GB storage?

    If so, wouldn’t that be a dubious way to beat limits?
    I would expect that the storage limits of shared mailboxes would be limited by the licensed mail user (Office Admin 50GB in this case).


    1. All shared mailboxes are limited to a max size of 50GB without a license and you get an unlimited number of shared mailboxes, But MS has indicated of late that is it going to crack down on ‘abuses’ of individual users having shared mailboxes as additional storage. For example MS now says ‘you can’t use a shared mailbox for archiving or journaling’. Do so at your own risk.


    2. Again per –

      8 Archive mailboxes can only be used to archive mail for a single user or entity for which a license has been applied. Using an archive mailbox as a means to store mail from multiple users or entities is prohibited. For example, IT administrators can’t create shared mailboxes and have users copy (through the Cc or Bcc field, or through a transport rule) a shared mailbox for the explicit purpose of archiving.


  2. For a shared mailbox we are trying to go beyond 50 GB. I have given the shared mailbox a Exchange Online 1 license and a Exchange Online Archiving add on license. I did this based on this blurb from Microsoft’s documentation on storage limits for Exchange Online:

    “To access a shared mailbox, a user must have an Exchange Online license, but the shared mailbox doesn’t require a separate license. Without a license, shared mailboxes are limited to 50 GB. To increase the size limit to 100 GB, the shared mailbox must be assigned an Exchange Online Plan 2 license or an Exchange Online Plan 1 license with an Exchange Online Archiving add-on license.”

    However the shared mailbox still shows a limit of 50GB. What am I doing wrong? I know E0 2 would fix it but we have extra EO1 and EO Archive licenses.



    Looks like they moved Business Premium out of the Unlimited Archive provided list since you wrote.. gosh it’s hard to keep up and make it clear to my customers, when it’s almost impossible to get clear myself! thanks heaps for trying though! I was almost convinced M365BP did include Unlimited Archive. Jury is still out here but that hyperlink implies no it does not.. I just have to use 150gb of email space to see.. oh well.. I suppose that isn’t that hard to do to be sure…


    1. The M365 Services description says:

      3 Microsoft 365 Business Premium includes Exchange Online Archiving subscription. Each Exchange Online Archiving subscriber initially receives 100 GB of storage in the archive mailbox. When auto-expanding archiving is turned on, additional storage is automatically added when the 100 GB storage capacity is reached.

      I would suggest that is more ‘correct’ documentation


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