I've noticed OACS core mechanisms, such as error reports are capable of sending emails as it was the user's own email itself, plus, they don't count to the quote defined by the email provider.
How's that possible?
How would I add more custom notifications like that?
to answer to the on-topic part of this question: this is not an OpenACS feature. It just means your SMTP server accepted to send an email on behalf of this user and every machine on the pahh from it to your email client let it pass through.
This was probably not counted in their quota because it was sent by your infrastructure rather than theirs.