Aus der RFC1893 <http://www.faqs.org/rfcs/rfc1893.html>:
2.X.X Success
Success specifies that the DSN is reporting a positive delivery
action. Detail sub-codes may provide notification of
transformations required for delivery.
X.1.0 Other address status
Something about the address specified in the message caused
this DSN.
X.1.5 Destination address valid
This mailbox address as specified was valid. This status
code should be used for positive delivery reports.
Also genau das, was Du wolltest.