Lookup for "550 5.7.9"
Permanent Error - This status code indicates a permanent issue and is unlikely to resolve itself. You should contact the Email Service Provider.
Basic Status Code Details (550)
Requested action not taken: mailbox unavailable [E.g., mailbox not found, no access]
Enhanced Status Code Details (5.7.9)
Authentication mechanism is too weak
The selected authentication mechanism is weaker than the server policy permits. Client should retry with a new authentication mechanism.