Fix clippy warnings (#967)

Cargo.toml

@@ -123,6 +123,9 @@ tokio1-boring-tls = ["tokio1", "boring-tls", "dep:tokio1_boring"]
 
 dkim = ["dep:base64", "dep:sha2", "dep:rsa", "dep:ed25519-dalek"]
 
+[lints.rust]
+unexpected_cfgs = { level = "warn", check-cfg = ['cfg(lettre_ignore_tls_mismatch)'] }
+
 [package.metadata.docs.rs]
 all-features = true
 rustdoc-args = ["--cfg", "docsrs", "--cfg", "lettre_ignore_tls_mismatch"]