MTA-STS Server VPS for Strict Mail Transport Security

Before MTA-STS, SMTP between MTAs was effectively opportunistic on TLS. An attacker who could rewrite STARTTLS responses on the wire could trivially downgrade traffic to cleartext, and the sending MTA had no policy way to refuse. MTA-STS fixes this by letting domain owners publish a policy that says, in effect, you must use TLS to talk to my mail server, and you must verify my certificate. The policy is published as a small file at a well-known URL, mta-sts.yourdomain.com slash dot well-known slash mta-sts dot txt, and announced via a DNS TXT record at _mta-sts.yourdomain.com. Sending MTAs cache the policy and refuse to deliver mail over downgraded connections. Combined with TLS-RPT for failure reporting, MTA-STS gives you operational visibility into who is or is not respecting your TLS posture, which is a meaningful upgrade over pre-MTA-STS opportunistic-only TLS.

Hosting MTA-STS correctly requires both a mail server and an HTTPS webserver that serves the policy file at a stable URL. Anubiz Host offshore VPS can host both on the same instance with Nginx serving the policy file behind a Let's Encrypt certificate, and Postfix as the mail server publishing its TLS configuration that matches what the MTA-STS policy declares. The policy file is small, around five lines, and declares the mode, enforce or testing, the matching mail server hostname, and the maximum age in seconds. The DNS TXT record declares the policy ID, which lets remote MTAs detect when you have updated the policy and re-fetch. Add TLS-RPT by publishing another TXT record at _smtp._tls.yourdomain.com pointing at a reporting mailbox, then aggregate the JSON reports remote MTAs send daily. Most failures will be obvious misconfigurations on the sending side, but occasional reports also surface real downgrade attempts on the wire.

MTA-STS is one of those standards where small mistakes silently cause delivery failures. Always deploy in testing mode first, monitor TLS-RPT reports for a few weeks, then promote to enforce only after you have evidence that legitimate senders successfully match your policy. Your certificate chain on the mail server hostname must match exactly what the MTA-STS policy declares. Wildcard certs work if your policy uses a wildcard host pattern, but in practice it is cleaner to use an explicit hostname like mx.yourdomain.com and pin that exactly. When rotating certificates, ensure the new chain is fully deployed before the old one expires. Modern Let's Encrypt automation handles this cleanly, but home-grown ACME scripts have caused real outages when a misconfigured rotation left an MTA-STS-enforced domain unable to receive mail for hours.