In Exchange 2003 not having all those excellent cmdlets can be frustrating. I have listed a couple examples here of how to get mailbox limit information using the Powershell, “QUEST AD Management Shell” cmdlets to gather mailbox limit information from AD user objects.

Get User Mailbox Limit Information from an OU.

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Get-QADUser -SearchRoot 'domain.com/users/' -includedProperties "mDBOverHardQuotaLimit","mDBOverQuotaLimit","mDBStorageQuota" | Select Name,Mail,@{N="Warning";E={$_.mDBStorageQuota}},@{N="HardLimit";E={$_.mDBOverQuotaLimit}} 
Get User Mailbox Information from an Text.txt file. 
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Get-Content C:\Users.txt | %{Get-QADUser $_ -includedProperties "mDBOverHardQuotaLimit","mDBOverQuotaLimit","mDBStorageQuota" | Select Name,Mail,@{N="Warning";E={$_.mDBStorageQuota}},@{N="HardLimit";E={$_.mDBOverQuotaLimit}}

Hope this helps.

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