How to Enable Exchange Online Mailbox Archives Based on Mailbox Size

Automatically Enable Archive Mailboxes Once the Primary Mailbox Exceeds a Threshold

A question following my article about how to transition from Exchange Online mailbox retention policies to Microsoft Purview retention policies asked:

“Is there a way in legacy or M365 online archiving policies , that it can be enabled based on primary mailbox data size ,say for example mailbox size crosses 40 gb , it’s online archive gets enabled automatically and older data gets move to online archive to keep primary mailbox at 40 gb limit.”

It’s a reasonable request. Essentially, the organization wants users to keep all email in their primary mailboxes until the mailboxes get to 40GB. Once that point is reached, the organization wants to enable archives for those mailboxes and start to move old email from the primary mailboxes to the archives to keep the size of the primary under 40 GB.

Archive Mailboxes and Sizing

These are proper archive mailboxes and not Outlook’s archive folder. Real archive mailboxes can grow to up to 1.5 TB using the Exchange Online auto-expanding mechanism. Note: if you enable auto-expanding archives, you cannot move those archive mailboxes back to an on-premises Exchange server.

Exchange Online enterprise mailboxes have quotas of between 50 GB and 100 GB based on the license assigned to the account, so the 40 GB threshold is a tad arbitrary. It might be that keeping under this size assures reasonable performance for the OST file. If so, that’s a good thing because you don’t want the OST to become so large that it impacts PC performance.

Assigning Archives Based on Mailbox Size

The outline of the solution is:

1. Find mailboxes that are not archive-enabled.
2. Check the mailbox size.
3. If the mailbox size exceeds the threshold, enable the archive mailbox, and assign an Exchange Online mailbox retention policy to instruct the Mailbox Folder Assistant to move items from the primary to the archive mailbox after they reach a certain age.

Exchange Online mailbox retention policies are the only way to move items into an archive mailbox. Microsoft Purview retention policies can keep or remove items, but they cannot move mailbox items.

To prepare, I created an Exchange Online mailbox retention policy with a single default move to archive tag (Figure 1). The policy can contain other retention tags to handle processing of default folders like the Inbox and Sent Items, or to allow users to mark items for retention. However, all that we need is the default move to archive tag. In this instance, the tag instructs the MFA to move items from the primary to the archive mailbox once they reach 730 days (2 years) old.

Now we need some PowerShell to check for and process mailboxes. Here’s the script that I came up with:

# Define archive threshold
$ArchiveThreshold = 40GB

# Find mailboxes without an archive
Write-Host "Looking for mailboxes that are not archive-enabled..."
[array]$Mbx = Get-ExoMailbox -RecipientTypeDetails UserMailbox -Filter {ArchiveState -ne "Local"} -ResultSize Unlimited
If (!($Mbx)) { Write-Host "No mailboxes found without archives - exiting!" ; break }

Write-Host ("Checking {0} mailboxes" -f $Mbx.count); $MbxUpdated = 0
ForEach ($M in $Mbx) {
   
   $Stats = Get-ExoMailboxstatistics -Identity $M.ExternalDirectoryObjectId
   If ($Stats.TotalItemSize.Value -gt $ArchiveThreshold) { # Mailbox size is larger than the threshold
      Write-Host ("Enabling archive for mailbox {0}..." -f $M.UserPrincipalName)
      Enable-Mailbox -Archive -Identity $M.ExternalDirectoryObjectId
      Set-Mailbox -Identity $M.ExternalDirectoryObjectId -RetentionPolicy "Mailbox Two-Year Archive Policy"
      $MbxUpdated++
   } #End if
} #End ForEach Mbx

Write-Host ("All done. {0} mailboxes were processed and {1} were archive-enabled" -f $Mbx.Count, $MbxUpdated)

Wrapping Things Up

To complete the solution, we should arrange for the script to be run periodically to be sure that mailboxes receive archives once they exceed the threshold. The scheduler in Azure Automation is a great way to run scripts like this and the cost to execute scripts is very reasonable. V3.0 of the Exchange Online management module introduced support for Azure Automation managed identities so there’s no danger of compromise due to leaked credentials. Which is exactly how it should be.

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