Delivery Status Deferred:

Emails get deferred at Proofpoint Essentials,  if the destination SMTP server is down or if there is a temporary network issue with the receiving server.
In this case, receiving server should be investigated, may be looking at receiving server's log details should help identifying the issue with that server.

If the E-mails are getting deferred at Proofpoint Essentials end, these emails will be automatically retried once every 5 min for the first hour, then once per 30 min after that. There is no manual flush involved or supported in this case.