Microsoft Clamps Down on Auto-Expanding Archive Mailboxes

New 1 TB limit for Exchange Online Bottomless Archives

Listing Exchange Online Archive Mailboxes with PowerShell
Listing Exchange Online Archive Mailboxes with PowerShell

Microsoft announced a “a truly bottomless archive” for Exchange Online in June 2015. At the time, Microsoft said that “the unlimited archive storage…” was available to “our Office 365 Enterprise E3 and E4 (now E5) plans.” The roll-out of the technology had some challenges, but stabilized at the end of 2016. I haven’t been aware of any great problems since and tenants now have some pretty large multi-terabyte archive mailboxes.

It turns out that unlimited isn’t unlimited after all. Microsoft recently updated the Exchange Online Archiving Service Description to say “The unlimited archiving feature in Office 365 (called auto-expanding archiving) provides up to 1 TB of storage in archive mailboxes in Exchange Online.” The same limit is documented in Exchange Online limits, which says “Each user 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. Office 365 provides up to 1 TB of additional storage in an archive mailbox.” The limit is enforced by restricting the number of auxiliary archives to 20. The bottom has well and truly been reached!

Users who already have archives that exceed the 1 TB threshold won’t lose data. However, they won’t be able to expand their archive by adding any more auxiliary mailboxes. At least, they won’t when Microsoft moves from a paper-based limitation to imposing a block in code.

All Quiet on the Microsoft Front

Microsoft didn’t announce the change. A notification wasn’t posted in the Office 365 admin center, no press release was issued, and the information released for the Microsoft Ignite conference in Orlando this week is curiously mute on the point. The only conclusion is that Microsoft is embarrassed at having to retreat from a commitment made to customers in 2015 and emphasized multiple times since.

 No one told Satya that Exchange Online was limiting bottomless archive mailboxes before his keynote at Microsoft Ignite 2019
Whoops! No one told Satya that Exchange Online was limiting bottomless archive mailboxes before his keynote at Microsoft Ignite 2019

Why has this happened? I don’t know what circumstances convinced Microsoft to terminate unlimited archive storage. I suspect that it might be associated to the way that some migration tools use archive mailboxes as targets to import data from third-party systems like Enterprise Vault. Often there’s no problem as the migration moves information belonging to users from the legacy repository to their archive mailboxes, but issues do occur when the migration moves information for multiple users (for instance, data for ex-employees) to a single archive mailbox.

Migration to Archive Mailboxes

Microsoft makes their view clear about how people should use auto-expanding mailboxes: “Auto-expanding archive is only supported for mailboxes used for individual users (or shared mailboxes) with a growth rate that doesn’t exceed 1 GB per day. Using journaling, transport rules, or auto-forwarding rules to copy messages to an archive mailbox is not permitted. Microsoft reserves the right to deny unlimited archiving in instances where a user’s archive mailbox is used to store archive data for other users.

In other words, they don’t want tenants to import more than 1 GB of day into an archive mailbox (most migration products will move more than this amount daily) and they don’t want tenants to set up archive mailboxes that act as repositories for legacy data (shared by multiple users). In a nutshell, Microsoft views auto-expanding archives as a personal user-centric feature. For these reasons, if you’re involved in a migration project to move data to Exchange Online, ask your migration vendor how they handle the provision and population of target mailboxes.

Exchange Online is a Business

Microsoft is sensitive to what they see as unexpected or unanticipated use of cloud resources. Office 365 is a business that now serves 200 million monthly active users. It’s a major driver for Microsoft’s cloud revenues. But it costs to install and manage the 200,000 Exchange Online servers and storage is a major part of the cost envelope.

Enterprise users are assigned generous 100 GB limits along with another 100 GB for recoverable items plus the archive. Although Microsoft uses cheap JBOD for most of its Exchange Online storage, when you think about the amount of storage needed to accommodate truly bottomless archives, you can understand why Microsoft might act if signs exist that storage is consumed in unusual ways. After all, they cut unlimited storage for OneDrive in 2015. At the time it was reported that some were using OneDrive to store massive movie collections consuming over 75 TB.

Failure to Communicate

A solid business case might exist to limit auto-expanding archives, but that’s not the point. The problem here is that Microsoft utterly failed to communicate that they no longer support bottomless archives and why the strategy needed to change.

One TB is a reasonable archive storage limit for most Office 365 tenants and shouldn’t cause enormous problems in practice, but architects and administrators must know details of technical limitations to incorporate into their plans. Sneaking in changes like this without notification will erode customer faith in the way Microsoft manages Office 365 and that’s not a good thing.


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