The Hornetsecurity Continuity Service ensures access to your emails while the recipient mail server is unavailable. The service will deliver the emails once the server is available again.
First you will need to locate the email through the Email Live Tracking and open the SMTP tab. There you will find a reference to Continuity:
TLS - continutiyservice - 444 Redirect because:
As soon as an email has been made available through the Continuity Service it will keep its Deferred status, even if the server becomes available again and the email was successfully delivered. This is expected behavior, since the email was already processed through the Continuity Service.