Needed to check all users who had SEND AS Permissions for someone else in the Exchange Enviroment apart from themselves and e.g. BESADMIN or other service accounts. Below is the command line

Source code

Get-Mailbox -Resultsize Unlimited | Get-ADPermission | ? {($_.ExtendedRights -like "*send-as*") -and -not ($_.User -like "nt authority\self") -and -not ($_.User -like "Domain\BESAdmin") -and -not ($_.User -like "domain\user")} | ft Identity, User -auto

As per the track back , can be limited to a specific OU