Lookup for "550 2.7.5"
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 (2.7.5)
Cryptographic failure
A transport system otherwise authorized to validate or decrypt a message in transport was unable to do so because necessary information such as key was not available or such information was invalid.