Lookup for "553 2.3.4"
This status code indicates a successful delivery or other operation. Any details are simply notes.
Basic Status Code Details (553)
Requested action not taken: mailbox name not allowed [E.g., mailbox syntax incorrect]
Enhanced Status Code Details (2.3.4)
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.