Try:
New-ReceiveConnector -Name RelayConnector -usage Custom -Bindings ’192.168.50.3:25′ -fqdn server.domain.com -RemoteIPRanges 192.168.40.4 -server MYEXCHANGESERVER -permissiongroups ExchangeServers -AuthMechanism ‘TLS, ExternalAuthoritative’
And than:
Get-ReceiveConnector "RelayConnector" | Add-ADPermission -User "NT AUTHORITY\ANONYMOUS LOGON" -ExtendedRights "ms-Exch-SMTP-Accept-Any-Recipient"