If user is not a member of a site, Sakai currently rejects with a 450 which essentially tells the mail server on the other side to keep on trying:
(host 10.113.65.140[10.113.65.140] said: 450 Your message cannot be delivered because you are not a member of the site, or you are a member but don't have the permission to send email to the site, or because you are registered with a different email address. If you are sending email from the correct email address, and you believe your email should be accepted at the site please contact the site owner and have them check the permission settings for the email archive tool under 'Permissions' for that tool. (in reply to RCPT TO command))
I believe we should be sending a 550 to actually tell the mail server to stop trying ......