Skip to content
Spaw
Get your API key

MTA-STS

published · September 13, 2026

MTA-STS (RFC 8461) is a policy a domain publishes in DNS at _mta-sts.<domain> and over HTTPS that tells sending servers to deliver only over authenticated TLS to its named mail hosts.

MTA-STS, SMTP MTA Strict Transport Security, is the mechanism of RFC 8461 by which a receiving domain tells sending servers that mail for it must be delivered over TLS with a valid certificate to the mail hosts it names: a TXT record at _mta-sts.<domain> announces the policy's version, and the policy itself is fetched over HTTPS from mta-sts.<domain>. Without it, SMTP falls back to plain text whenever a connection cannot negotiate TLS, which is exactly what an attacker on the path arranges.

What Spaw answers

The domain endpoint reports mta_sts: whether the DNS record that announces a policy exists. True means the domain has published one; false is a checked negative; null means the lookup failed. The free checker reads it beside TLS-RPT and BIMI.

What it does not mean

The record's presence does not prove the policy file is reachable or valid, or that the named hosts match the MX records; a full validation fetches and parses the policy, which Spaw does not do. And a domain without MTA-STS is not insecure in itself — most mail still travels over opportunistic TLS — it is a domain that has not closed the downgrade.

Related

markdown version: /glossary/mta-sts.md

Verify addresses the same way

The Spaw API runs every check described here on each lookup, with an SMTP handshake that never sends mail. 10 free lookups a month, no card required.

Get your API key

More glossary

  • Area-code overlay

    An area-code overlay is a new area code added over the same area as an existing one, instead of splitting it, so neighbours can hold different codes and local calls need ten digits.

  • Calling window

    A calling window is the span of local hours a sender contacts people in. Spaw answers in_call_window from the number's time zones: clock arithmetic, not compliance advice.

  • Fictional number

    A fictional number sits in a range kept out of service for films, TV and other fiction: 555-0100 to 555-0199 in North America, and ranges Ofcom, ARCEP, ACMA and ComReg reserve elsewhere.

  • International (exit) prefix

    An international prefix is what is dialled to leave a country before the calling code: 00 in most of the world, 011 in North America. Spaw reads it and suggests a fix in did_you_mean.

  • International network number

    An international network number sits under a calling code the ITU gives a global service rather than a country, such as +870, +881, +882 or +883, so its country is null.

  • Live carrier check (HLR lookup)

    A live carrier check asks the mobile networks, at the moment of the lookup, whether a number is in service, which network serves it and whether it is roaming. On Spaw it is not enabled.

  • Number pooling (thousands-blocks)

    Number pooling splits a North American central office code into ten blocks of a thousand numbers, each assigned to a carrier separately, so one code can hold several carriers' numbers.

  • Number portability

    Number portability lets a subscriber keep a phone number when changing carrier. The number's block stays listed under the carrier it was issued to, so block data names the original holder.