STARTTLS
Also known as: Opportunistic TLS
STARTTLS is an email protocol command that upgrades a plain text connection to an encrypted one (TLS/SSL).
STARTTLS is the command used to secure email in transit. Unlike standard web traffic (HTTPS) which is encrypted from the start, email connections often begin in plain text. The sending server issues the STARTTLS command to ask, "Can we talk securely?"
If the receiving server supports it, the connection is upgraded to use Transport Layer Security (TLS), encrypting the email so it cannot be read if intercepted. If the receiver doesn't support it, the email is usually sent in plain text (unencrypted), unless policies like MTA-STS prevent it.
Today, the vast majority of email traffic is encrypted via STARTTLS.
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