Fix WebPKIError(UnknownIssuer) when sending with rustls

Sending would return Err:
    Io(Custom { kind: InvalidData, error: WebPKIError(UnknownIssuer) })

src/smtp/mod.rs

@@ -27,6 +27,8 @@ use log::{debug, info};
 use native_tls::{Protocol, TlsConnector};
 #[cfg(feature = "rustls")]
 use rustls::ClientConfig;
+#[cfg(feature = "rustls")]
+use webpki_roots::TLS_SERVER_ROOTS;
 use std::net::{SocketAddr, ToSocketAddrs};
 use std::time::Duration;
 
@@ -158,7 +160,10 @@ impl SmtpClient {
 
     #[cfg(feature = "rustls")]
     pub fn new_simple(domain: &str) -> Result<SmtpClient, Error> {
-        let tls_parameters = ClientTlsParameters::new(domain.to_string(), ClientConfig::new());
+        let mut tls = ClientConfig::new();
+        tls.config.root_store.add_server_trust_anchors(&TLS_SERVER_ROOTS);
+
+        let tls_parameters = ClientTlsParameters::new(domain.to_string(), tls);
 
         SmtpClient::new(
             (domain, SUBMISSIONS_PORT),