lettre is a a mailer library for Rust. Starting in version 0.10.1 and prior to version 0.11.22, an inverted-boolean bug in lettre's "boring-tls" integration silently disables TLS hostname verification for callers using the default (strict) configuration. An on-path attacker presenting any chain-valid certificate for any domain can intercept SMTP submission, including PLAIN/LOGIN credentials and message contents, against any lettre user built with the "boring-tls" feature. Other TLS backends ("native-tls", "rustls") are unaffected. Version 0.11.22 patches the issue.