I wrote this quick and dirty one-liner to help monitor and find stuck emails in Exchange / Office365 mailboxes across a tenant and export the list as a CSV.

Get-Mailbox -ResultSize Unlimited | Get-MailboxFolderStatistics | Where-Object {$_.Name -eq "Outbox" -and $_.ItemsInFolder -gt '0' } | Select-Object Identity, FolderType, ItemsinFolder, FolderSize | Export-CSV "C:\CSVs\Outbox.csv"