Lookup for "450 5.3.4"
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 (450)
Requested mail action not taken: mailbox unavailable [E.g., mailbox busy]
Enhanced Status Code Details (5.3.4)
Message too big for system
The message is larger than per-message size limit. This limit may either be for physical or administrative reasons. This is useful only as a permanent error.