This is the Mailhardener MTA-STS (RFC8461) policy service.
You can find the MTA-STS policy file for mailhardener.com here.
Message: The requested URL is not an MTA-STS policy file
MTA-STS is an email security mechanism to prevent email messages from being intercepted during transit.
Mailhardener offers hosted MTA-STS services to allow any domain to use MTA-STS.
Visit mailhardener.com for more information.