true. i sent an email with a fake acount to a valid recipient address with CC to my gmail account and in fact, i got the relay related error as you described.
but, and here is my concern, i used an external machine remotely, to send an email to me. i created an email account with a fake sender but a valid recipient and the mail was delivered to my mailbox.
What's your opinion about secure a little more the spamsnake adding SASL?