Lookup for "501 4.3.1"
Temporary Error - This status code indicates a transient issue and may resolve itself.
Basic Status Code Details (501)
Syntax error in parameters or arguments
Enhanced Status Code Details (4.3.1)
Mail system full
Mail system storage has been exceeded. The general semantics imply that the individual recipient may not be able to delete material to make room for additional messages. This is useful only as a persistent transient error.