Thank You All. We used below one to get the total Inbox & Sent Items at organization level
(Inbox)
$mailboxes = @(Get-Mailbox -ResultSize Unlimited)
[Int] $inboxstats = 0
foreach ($mailbox in $mailboxes)
{
$inboxItems = Get-MailboxFolderStatistics $mailbox -FolderScope Inbox | Where {$.FolderPath -eq "/Inbox" -and $.FolderType -eq "Inbox"}
$inboxstats += $inboxItems.ItemsinFolderandSubfolders
}
Write-Host "Organization Inbox Items: $inboxstats" -ForegroundColor DarkGreen
(Sent Items)
$mailboxes = @(Get-Mailbox -ResultSize Unlimited)
[Int] $sentitemsstats = 0
foreach ($mailbox in $mailboxes)
{
$sentItems = Get-MailboxFolderStatistics $mailbox -FolderScope SentItems | Where {$.FolderPath -eq "/Sent Items" -and $.FolderType -eq "SentItems"}
$sentitemsstats += $sentItems.ItemsinFolderandSubfolders
}
Write-Host "Organization Sent Items: $sentitemsstats" -ForegroundColor DarkGreen