Lookup for "538 4.2.1"
Temporary Error - This status code indicates a transient issue and may resolve itself.
Basic Status Code Details (538)
Encryption required for requested authentication mechanism
Enhanced Status Code Details (4.2.1)
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.