Get sum of items, deleted items, total item sizes, and deleted item sizes for all specified mailboxes

Get-MailboxItemTotal.ps1

function Get-MailboxItemTotal {
    [CmdletBinding()]
    param(
        $Identity
    )

    PROCESS {
        if ($Identity) {
            Write-Verbose 'Getting specified mailboxes.'
            $Identities = $Identity | Foreach-Object {Get-Mailbox -Identity $_} | Select-Object -ExpandProperty Identity
        } else {
            Write-Verbose 'Getting all mailboxes.'
            $Identities = Get-Mailbox -ResultSize Unlimited | Select-Object -ExpandProperty Identity
        }
        Write-Verbose 'Getting statistics.'
        $ItemStats = foreach ($id in $Identities) {
            Get-MailboxStatistics $id | Select-Object -Property @(
                'ItemCount'
                @{ Name = 'TotalItemSize'; Expression = { $_.TotalItemSize.Value.ToBytes() } }
                'DeletedItemCount'
                @{ Name = 'TotalDeletedItemSize'; Expression = { $_.TotalDeletedItemSize.Value.ToBytes() } }
            )
        }

        $Properties = @{
            TotalItemCount        = ($ItemStats | Measure-Object -Property ItemCount -Sum).Sum
            TotalItemSize         = ($ItemStats | Measure-Object -Property TotalItemSize -Sum).Sum / 1TB
            TotalDeletedItemCount = ($ItemStats | Measure-Object -Property DeletedItemCount -Sum).Sum
            TotalDeletedItemSize  = ($ItemStats | Measure-Object -Property TotalDeletedItemSize -Sum).Sum / 1TB
        }

        New-Object -TypeName PSCustomObject -Property $Properties
    }
}