Add the "MTA-STS" note.

Author: paveloom

content/notes/MTA-STS.md

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+title = "MTA-STS"
++++
+
+A quote from [RFC 8461](https://datatracker.ietf.org/doc/html/rfc8461):
+
+> SMTP MTA Strict Transport Security (MTA-STS) is a mechanism enabling mail service providers (SPs) to declare their ability to receive Transport Layer Security (TLS) secure SMTP connections and to specify whether sending SMTP servers should refuse to deliver to MX hosts that do not offer TLS with a trusted server certificate.
+
+Links:
+
+- [RFC 8461](https://datatracker.ietf.org/doc/html/rfc8461)
+
+## Setup
+
+The setup essentially boils down to two steps:
+
+1. Add a TXT DNS record:
+
+   ```
+   _mta-sts.example.com.  IN TXT "v=STSv1; id=20160831085700Z;"
+   ```
+
+2. Add a policy at `https://mta-sts.example.com/.well-known/mta-sts.txt`:
+
+   ```
+   version: STSv1
+   mode: enforce
+   max_age: 604800
+   mx: mail.example.com
+   ```