Fix 1.59 clippy warnings

src/smtp.rs

@@ -246,7 +246,7 @@ pub(crate) async fn smtp_send(
                             // Other enhanced status codes, such as Postfix
                             // "550 5.1.1 <foobar@example.org>: Recipient address rejected: User unknown in local recipient table"
                             // are not ignored.
-                            response.first_word() == Some(&"5.5.0".to_string())
+                            response.first_word() == Some("5.5.0")
                         }
                         _ => false,
                     };