Lookup for "501 2.2.1"
This status code indicates a successful delivery or other operation. Any details are simply notes.
Basic Status Code Details (501)
Syntax error in parameters or arguments
Enhanced Status Code Details (2.2.1)
Mailbox disabled, not accepting messages
The mailbox exists, but is not accepting messages. This may be a permanent error if the mailbox will never be re-enabled or a transient error if the mailbox is only temporarily disabled.