// MTA-STS policy host

This host exists to serve one file: the MTA-STS policy for seqontrol.com, at /.well-known/mta-sts.txt. It tells sending mail servers that mail for seqontrol.com must travel over TLS to a published MX, so a network attacker cannot strip the encryption and read it.

It is a separate host because the standard says so: senders fetch the policy from mta-sts.<domain> over HTTPS, never from the domain itself.

Nothing else is served here.