Get-MailboxSenders

function Get-MailboxSenders {

<#
    .Synopsis
    Queries the allowed senders of a mailbox

    .DESCRIPTION
    Queries the senders who are allowed to send as a specified mailbox

    .EXAMPLE
    Get-MailboxSenders -Mailbox Charles.Curtis
#>

  param (
    [Parameter(Mandatory=$true,HelpMessage='You must specify a mailbox')][string]$Mailbox
    )
  if(-not($Mailbox)) { Throw 'You must supply a mailbox to search' }

  Import-Module ActiveDirectory

  $ErrorActionPreference = 'Stop'
  try {$UserMailbox = Get-Mailbox $Mailbox -ErrorAction Stop }
  catch {
    Write-Error 'Could not find mailbox'
    }

  $alias = $UserMailbox.alias
  $senders = Get-Mailbox $UserMailbox.Alias | Get-ADPermission | Where-Object {($_.ExtendedRights -like '*send-as*') -and -not ($_.User -like '*nt authority*') -and -not ($_.User -like "*$alias") -and ($_.IsInherited -eq $false)}
  if ($senders.Count -lt '1'){
    Write-Host 'No senders were found' -ForegroundColor Yellow
  }
  foreach ($sender in $senders) {
    Get-ADUser $sender.User.SecurityIdentifier.ToString() -Properties *
    #Get-ADUser $account -Properties *
   }
}