# TLS policy

Bongo offers TLS 1.3 and TLS 1.2 by default. Older protocol versions are only
available on listeners whose explicit legacy-TLS option is enabled. Keep those
listeners restricted to trusted legacy networks; do not expose them directly
to the Internet.

The SMTP delivery agent uses opportunistic STARTTLS. When a peer offers
STARTTLS, `outbound_tls_verify` verifies its certificate chain against the
system trust store and verifies the connected MX or configured LMTP host name.
It defaults to `true`. Set it to `false` only for a peer whose certificate
cannot yet be repaired.

`outbound_tls_required` defaults to `false`, so delivery remains possible to
SMTP servers which do not offer STARTTLS. Enabling it defers mail instead of
sending plaintext. It is a global policy and should only be enabled when every
configured destination is known to support TLS. A later policy layer may apply
per-domain DANE or MTA-STS requirements.

External-account collection and submission use libcurl and independently
verify certificates and host names by default. See `external-accounts.md` for
the credential and provider policy.