Lookup for "550 2.1.5"
This status code indicates a successful delivery or other operation. Any details are simply notes.
Basic Status Code Details (550)
Requested action not taken: mailbox unavailable [E.g., mailbox not found, no access]
Enhanced Status Code Details (2.1.5)
Destination address valid
This mailbox address as specified was valid. This status code should be used for positive delivery reports.