Since around 1pm all emails to my domain have been bounced undelivered with the following message:

"Final-Recipient: rfc822; *** Email address is removed for privacy ***
Action: failed
Status: 5.0.0
Remote-MTA: dns; ci-ni.org.uk (50.115.23.71, the server for the domain.)
Diagnostic-Code: smtp; 550-The mail server could not deliver mail to *** Email address is removed for privacy ***.  The account
 550-or domain may not exist, they may be blacklisted, or missing the proper dns
 550 entries.
Last-Attempt-Date: Wed, 25 Jul 2018 05:36:48 -0700 (PDT)"

Can you help me fix this issue? Many thanks,

Roger