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Office 365 – Is the “Archive Mailbox” Still Relevant?

The practice of email “archiving” probably goes back as far as email itself. Over the years, the reason for archiving emails has changed and as the corresponding technologies have evolved, it brings into question whether this practice even needs to continue.
Before enabling functionality that moves email data from it’s original location, it’s worth discussing if it is even necessary. To do so, we should ask the following questions:
Why do we archive?
Does this help or hurt our end-user experience?
Does this benefit our legal requirements?

The article below provides some history around the practice of email archiving and why it may no longer be relevant.


When discussing “archiving” with clients, I find that different organizations have different definitions around this term. For the context of this article, I am discussing the archiving functionality built into Exchange Online. This is different than a third-party system that may use something like “journaling” to copy messages into an external data store or use “stubbing” technology to place emails into a “read-only” data store.


Starting the conversation within the current millennium, you likely had a mailbox on Exchange 2003 with a quota of no more than 200 MB – 500 MB. Storage was expensive and performance became an issue for both the client and the server when mailboxes became excessively large. So administrators set quotas and users were forced to manage their mailbox size by either deleting messages or moving them somewhere else outside of the mailbox. While administrators may have implemented Mailbox Manager policies, how the users got the mailbox below the quota was often left up to them. Perhaps they removed messages by age or perhaps by size; where the data went was also left up to the user, it may have been deleted or it may have been moved into a PST file. The sneakiest of users took that PST and put it on the file server, basically just moving the data from the email server to the file server.
In Exchange 2007, we had Messaging Records Management (MRM) which had varied levels of deployment but no real answers on what to do with the data and this is about the time that it seemed like most of the third-party archiving products were being deployed. Exchange 2010 brought forth another change with the switch to Email Retention Policies but now we also had this new concept of an “archive mailbox”.

The Archive Mailbox

Whether you refer to is as the “Personal Archive”, “Online Archive” or “In-Place Archive”, the archive mailbox is basically just a second mailbox for your user. Here are the two main differences between the archive mailbox and the primary mailbox:

  • The archive mailbox is not cached as part of the OST
  • The archive mailbox has an unlimited capacity for users with E3 licenses

Important to understand here is that the archive mailbox is just like the user’s primary mailbox from an email retention standpoint, the archive mailbox is not “read-only” as is sometimes misunderstood.

Why Use the Archive Mailbox?

Looking at the above differences between the archive mailbox and the primary mailbox can lead us to the discussion of why the archive mailbox might be used:
The OST cache created by Outlook can have an impact on the workstation when it becomes excessively large. Since the archive mailbox is not cached, it makes sense that keeping the primary mailbox data to something like 90 days or one year would keep the OST size manageable. This was a practice that was used for a while, until Outlook 2013 that is. Now that Outlook 2013 allows us to control the amount of data cached, we can achieve the same goal from the client side. Not running Outlook 2013? Well you should be; in addition to this feature and others that you’re missing out on, Outlook 2010 will come off mainstream support in October of this year.
Office 365 allows for a 50 GB mailbox; this limit was 25 GB only a couple years ago which may have made the archive mailbox more relevant back then. A 50 GB mailbox is likely a larger mailbox than we’ve been allowed to have in our on-premises Exchange with few exceptions. When 50 GB just isn’t enough and you “need” to keep all those messages, the archive mailbox allows for unlimited email hoarding. Important to understand is that the “unlimited” archive is an E3 feature. While the E1 licenses allow for an archive mailbox, the capacity of the primary mailbox plus the archive mailbox is limited to a combined total of 50 GB for E1 users. So moving messages from the primary mailbox to the archive mailbox with an E1 license does not give you any additional storage capacity.
Other Reasons
Aside from the above, you will have administrators that feel the need to force their users into keeping a “clean mailbox”. So everything under two years is in the primary mailbox and everything older is in the archive mailbox. Yes, we can search them both so the user should be able to find their emails but I think it’s important to question what value this really adds. The concept of a “clean mailbox” is certainly subjective and could come at the cost of user confusion as to where their emails are.

But I Need to Keep Emails for Compliance Reasons

Office 365 certainly offers the ability to make mailbox items immutable but archiving is not the feature that does this. If you’re looking to ensure that emails are kept for a certain period of time, you want to look at Litigation Hold or In-Place Hold. These are the features that will ensure that an emails is retained even if the user “deletes” it from their view of the mailbox.

Retention Policies

Related to archive mailboxes, I find that the concept of “Retention Policies” are often misunderstood. Within Office 365 we can do use Retention Policies to do things like move messages to the archive mailbox after X days or force the deletion of an item after X days. Important to understand here is that these are the maximum number of days that the item is kept. If I set a retention policy to delete items after 365 days, there is no guarantee that the item will be kept that long, the user may have deleted it after 100 days. If you need to ensure that messages are kept a certain period of time, that’s where you want to use the above mentioned Litigation Hold or In-Place Hold. If the goal is to only have emails three years old in your mailbox, then you will want to use a combination of Retention Policies and Litigation Hold / In-Place Hold.
Also occasionally misunderstood is the concept of “Retention Hold“. This feature just places your retention policies on pause so if someone is absent for an extended period of time, action is not taken on that person’s emails before they get a chance to even look at them.


  • Use the E3 archive mailbox when there are capacity constraints with the primary mailbox
  • Outlook 2013 can limit the OST cache size of the primary mailbox
  • Litigation Hold and In-Place Hold define the minimum time an item will be kept
  • Retention Policies define the maximum time an item can be kept

Did you find this article helpful?
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Thoughts on “Office 365 – Is the “Archive Mailbox” Still Relevant?”

  1. Erica Castillo

    Thanks for this article! I’m currently working on getting our Archiving Mailbox set up, but am running into some confusion. I’m hoping you can help clarify.
    We currently have our “Retention Policy” set to unlimited on the main mailboxes for users, meaning we are not deleting any content in mailboxes.
    We are planning to change this to possibly 5 years, instead. If we set up the “archiving mailbox”, will that be the same for that mailbox? Since my understanding is that you can only have 1 retention policy per mailbox and the “archive mailbox” is considered to be part of the main mailbox.
    Can you confirm my thinking:
    Life of main mailbox- User deletes items-> sent to Deleted Items Folder (with retention policy of eg 30 days) or user deletes the Deleted Items folder-> These items are sent to the Recoverable Items Folder (with retention policy of eg. 5 years). Once the 5 years are up, those items are purged/ disappear irrecoverably unless you place the mailbox on In Place Hold or Litigation Hold.
    Life of Archive Mailbox- Once messages are sent to this folder (determined by Auto-Archive rule? eg. 2 years), those messages will remain in this folder for an additional 3 years (to give the total life cycle of the content to 5 years?). So, even if user deletes items in the Archive Mailbox, they will remain in the Recoverable Items Folder (within the Archive Folder) for those 3 years?
    Sorry if this didn’t make sense, but hoping you can help clarify for me.
    Thank you!

  2. Great post! Clarified a lot of confusion I had and saved me more wasted time trying to configure archiving and make retention policies (What I really needed was In-Place holds).

  3. Great article, thanks.
    As someone who was not actively using Outlook/Exchange in the days of storage struggle, you cleared out the whole Archiving feature.
    Since Archive is not cached in OST it still sounds really useful for huge shared mailboxes that have performance issues, without reducing cached period size

  4. Hi Joe, great article! You state “While the E1 licenses allow for an archive mailbox, the capacity of the primary mailbox plus the archive mailbox is limited to a combined total of 50 GB for E1 users.” It appears that this has changed to 50+50 can you confirm?

  5. Just a heads up, Microsoft is no longer expanding Archives. They say their is an Auto-expansion feature being rolled out “soon”. So far after several months of requesting updates and getting nothing but “we don’t know exactly” they are violating their own policy (from what I recall) and recommending using an additional mailbox for archiving. Advertised Unlimited and get this instead. Truly feel frustrated with this.

  6. Unfortunately it seems like the auto-archive feature in Outlook has not been updated to keep up with the times, as it still needs to use a PST to auto-archive. Is there any feature to auto-archive to another folder tree or an Archive mailbox from the client site?

  7. This post seems to be out of date. Office 365 revamped the compliance section and now data retention is set by retention policy’s.
    Based on my interpretation of the site below: When a user deletes a email, SharePoint item, one drive item, public folder or 365 group data its put in a recoverable items folder until it hits the retention rule limits.
    I welcome any feedback as I may have miss understood the content..
    Also useful is this site:

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