Glossary
STARTTLS
An SMTP service extension that lets a client and server upgrade an existing plaintext connection to TLS. The server advertises it as an EHLO keyword and the client invokes it with the STARTTLS command.
In plain terms
The handshake that turns an unencrypted mail connection into an encrypted one.
Why it matters for deliverability
STARTTLS is opportunistic by default: if the upgrade fails, mail is usually still delivered in the clear. That gap is exactly what MTA-STS exists to close.
What to do about it
- Offer STARTTLS on every receiving host with a valid certificate.
- Monitor how much of your outbound mail is actually delivered over TLS.
- Publish MTA-STS if you need TLS to be enforced rather than merely attempted.
Source
RFC 3207 — SMTP Service Extension for Secure SMTP over TLS
“The STARTTLS keyword is used to tell the SMTP client that the SMTP server is currently able to negotiate the use of TLS.”
Checked 2026-08-29
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