Lookup for "501 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 (501)
Syntax error in parameters or arguments
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.