Lookup for "503 2.3.2"
This status code indicates a successful delivery or other operation. Any details are simply notes.
Basic Status Code Details (503)
Bad sequence of commands
Enhanced Status Code Details (2.3.2)
System not accepting network messages
The host on which the mailbox is resident is not accepting messages. Examples of such conditions include an immanent shutdown, excessive load, or system maintenance. This is useful for both permanent and persistent transient errors.