Calculating size of user’s mailbox and any delegated mailboxes

Outlook by default limits OST to 50GB (modern versions) but some users may have tons of delegated mailboxes and run into this limit. This script retrieves users that have more than 50GB of delegated and personal mailboxes visible. You might not want to increase OST limit for everyone…

Possible use case is situation where you have delegated several large mailboxes to multiple users. As tickets start coming in as mailboxes grow, you want to proactively find out problematic users.

This really becomes an issue when you delegate mailboxes to groups. I’ll post script to update msExchDelegateListBL for group memberships in a few days as Exchange doesn’t do that automatically. TL;DR: If you delegate mailbox to group, it doesn’t get autoloaded by Outlook. I have a script to remediate that.


  • This is a slow and ugly one-off. But as I only needed it once, it just works. As always, read the disclaimer on the left.
  • You need Exchange Management Tools installed on your PC. It doesn’t work with remote management PowerShell session as you don’t have proper data types loaded. Install management tools on your PC and run Exchange Management Shell.
  • This script looks up only admin-delegated mailboxes. Any folders or mailboxes or public folders shared and loaded by users themselves are not included. This is server-side view only.
$userlist = get-aduser -Filter *
foreach ($user in $userlist) {
	$usermailbox = get-mailbox $user.distinguishedname 2>$null
	If ($usermailbox) {
		$DelegationList = (get-aduser -Identity $user.distinguishedname -Properties msExchDelegateListBL).msExchDelegateListBL
		If ($DelegationList) {
			$usermailboxsize = (Get-mailboxstatistics -identity $usermailbox | select @{label=”TotalSizeBytes”;expression={$_.TotalItemSize.Value.ToBytes()}}).TotalSizeBytes
			$SharedSize = ($DelegationList | %{get-mailbox -Identity $_ | Get-MailboxStatistics | select displayname,@{label=”TotalSizeBytes”;expression={$_.TotalItemSize.Value.ToBytes()}},totalitemsize} | measure -sum totalsizebytes).sum
			$TotalVisibleSize = ( ($usermailboxsize + $SharedSize) / 1GB)
			If ($TotalVisibleSize -gt 50) {
				Write-Host $user.Name
				Write-Host $TotalVisibleSize

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