Repository: lettre/lettre (language: rust)

Issue:
UnknownIssuer error after commit 69e5974 with rustls
**Describe the bug**
Sending a mail using `smtp.gmail.com` with 0.11 results in an Unknown Issuer error when using rustls after commit 69e59740245b03f9588251450f1a7daa9409d436.
Using commit 4fb67a7da1a415696f29f167d9869c66e5ca5058 works without any issues.

**To Reproduce**
```rust
use lettre::message::header::ContentType;
use lettre::transport::smtp::authentication::Credentials;
use lettre::{AsyncSmtpTransport, AsyncTransport, Message, Tokio1Executor};

#[tokio::main]
async fn main() {
    let credentials = Credentials::new(/* username */, /* password */);

    let pool: AsyncSmtpTransport<Tokio1Executor> =
        AsyncSmtpTransport::<Tokio1Executor>::starttls_relay("smtp.gmail.com")
            .expect("Could not create SMTP pool")
            .credentials(credentials)
            .build();

    pool.send(
        Message::builder()
            .from(/* from@test.com */)
            .to(/* to@test.com */)
            .subject("Subject")
            .header(ContentType::TEXT_PLAIN)
            .unwrap(),
    )
    .await
    .unwrap();
}
```

```yaml
[dependencies]
tokio = { version = "1", features = ["full"] }
lettre = { version = "0.11.0", default-features = false, features = ["tokio1", "tokio1-rustls-tls", "tracing", "pool", "smtp-transport", "builder"] }
```

**Expected behavior**
Sent the email without error

**Environment (please complete the following information):**
- Lettre version: 0.11.0
- OS: 6.5.5-1-MANJARO


Discussion / hints:
I just hit the same issue, reverting to v0.10.4 worked. (I tought I reverted to 0.10.3, but didn't pin it in properly Cargo.toml, so cargo actually used 0.10.4 instead. It worked anyway, so it was not the rustls version update)
Experiencing this as well

```toml
lettre = { version = "0.11.0", default-features = false, features = [
  "tokio1",
  "rustls-tls",
  "tokio1-rustls-tls",
  "smtp-transport",
  "tracing",
  "builder",
] }
```
I can confirm  
```toml
[dependencies.lettre]  
version = "0.10.4"  
features = [ "tokio1-rustls-tls", "builder","smtp-transport" ]  
default-features = false  
```
OS: Windows 11

## Extra Info that could help
TLS Certificate was issued by Lets Encrypt 
Email Server: Stalwart Email Server https://stalw.art/

Fix patch:
diff --git a/src/transport/smtp/client/tls.rs b/src/transport/smtp/client/tls.rs
index 4894371f2..cab90356a 100644
--- a/src/transport/smtp/client/tls.rs
+++ b/src/transport/smtp/client/tls.rs
@@ -115,7 +115,7 @@ pub enum CertificateStore {
     /// Use a hardcoded set of Mozilla roots via the `webpki-roots` crate.
     ///
     /// This option is only available in the rustls backend.
-    #[cfg(feature = "webpki-roots")]
+    #[cfg(feature = "rustls-tls")]
     WebpkiRoots,
     /// Don't use any system certificates.
     None,
@@ -384,9 +384,9 @@ impl TlsParametersBuilder {
                 Ok(())
             }
 
-            #[cfg(feature = "webpki-roots")]
+            #[cfg(feature = "rustls-tls")]
             fn load_webpki_roots(store: &mut RootCertStore) {
-                store.add_server_trust_anchors(webpki_roots::TLS_SERVER_ROOTS.0.iter().map(|ta| {
+                store.add_server_trust_anchors(webpki_roots::TLS_SERVER_ROOTS.iter().map(|ta| {
                     rustls::OwnedTrustAnchor::from_subject_spki_name_constraints(
                         ta.subject,
                         ta.spki,
@@ -399,10 +399,10 @@ impl TlsParametersBuilder {
                 CertificateStore::Default => {
                     #[cfg(feature = "rustls-native-certs")]
                     load_native_roots(&mut root_cert_store)?;
-                    #[cfg(all(not(feature = "rustls-native-certs"), feature = "webpki-roots"))]
+                    #[cfg(not(feature = "rustls-native-certs"))]
                     load_webpki_roots(&mut root_cert_store);
                 }
-                #[cfg(feature = "webpki-roots")]
+                #[cfg(feature = "rustls-tls")]
                 CertificateStore::WebpkiRoots => {
                     load_webpki_roots(&mut root_cert_store);
                 }


Test patch: