Export Mailbox Statistics
Get mailbox statistics and export them to a CSV file. Use the Connect-ExchangeOnline function to use this code.
Get-Mailbox -RecipientTypeDetails UserMailbox | ForEach-Object { Get-MailboxStatistics $_.PrimarySmtpAddress } | Select DisplayName, ItemCount, TotalItemSize | Export-Csv -Path "MailboxStatistics.csv" -NoTypeInformation rCredential