I’ve been troubleshooting an issue where meeting requests are being received by mailboxes/users that are not included in the requests. To get a list of calendar delegates for mailboxes where these are set, I found the following couple of lines quite handy:

$output = [System.Collections.ArrayList]@()
foreach ($_mbx in (Get-Mailbox -ResultSize Unlimited | Get-CalendarProcessing | Where-Object { $_.ResourceDelegates -notlike [string]::Empty })) { 
    $_mbx.ResourceDelegates | %{ 
            User = $_mbx.Identity; 
            Delegate = $_.ToString() 
        })) | Out-Null