bongo/docs/tls.md

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.