# Encrypt with TLS

To transmit secured, authenticated, and verified communications between two or more systems, Twilio Email uses the symmetric cryptography of the [Transport Layer Security (TLS)][tls].

Four RFCs define TLS:

* v1.0 in [RFC 2246][rfc2246]
* v1.1 in [RFC 4346][rfc4346]
* v1.2 in [RFC 5246][rfc5246]
* v1.3 in [RFC 8446][rfc8446]

When attempting to deliver email, Twilio Email opportunistically tries outbound [TLS v1.1][rfc4346] or later.

If your recipient's email server accepts an inbound connection using TLS v1.1 or later, Twilio Email encrypts then delivers the email message between systems. If the server doesn't support TLS, Twilio delivers the message unencrypted.

[tls]: /docs/glossary/what-is-transport-layer-security-tls

[rfc2246]: https://datatracker.ietf.org/doc/html/rfc2246

[rfc4346]: https://datatracker.ietf.org/doc/html/rfc4346

[rfc5246]: https://datatracker.ietf.org/doc/html/rfc5246

[rfc8446]: https://datatracker.ietf.org/doc/html/rfc8446