the 550 error that you are getting means that spamass-milter is redirecting the emails back to the sender. Since the email is not a valid email account, I suggest you to discard or have it redirected to another email address (Scalix user called Spam or whatever) spam@localhost and then use this mailbox for sa-learn.
you can add options to you spamass-milter by adding the following flags/options
-B spam@localhost.localdomain -- the spam email will be sent to the original user plus a copy to the spam@localhost.localdomain
-b spam@localhost.localdomain - the spam email will only be sent to the original user
-r nn -- means to reject all emails with nn score. I had mine lowered to 7 instead of the 15 default value.