Add generateResponse without mail Session

acme4j-smime/src/main/java/org/shredzone/acme4j/smime/email/ResponseGenerator.java

@@ -17,11 +17,14 @@ import static java.util.Objects.requireNonNull;
 import static jakarta.mail.Message.RecipientType.TO;
 import static org.shredzone.acme4j.smime.email.ResponseBodyGenerator.RESPONSE_BODY_TYPE;
 
+import java.util.Properties;
+
 import edu.umd.cs.findbugs.annotations.Nullable;
 import jakarta.mail.Address;
 import jakarta.mail.Message;
 import jakarta.mail.MessagingException;
 import jakarta.mail.Session;
+import jakarta.mail.Transport;
 import jakarta.mail.internet.InternetAddress;
 import jakarta.mail.internet.MimeMessage;
 
@@ -108,6 +111,18 @@ public class ResponseGenerator {
         return this;
     }
 
+    /**
+     * Generates the response email.
+     * <p>
+     * A simple default mail session is used for generation.
+     *
+     * @return Generated {@link Message}.
+     * @since 2.16
+     */
+    public Message generateResponse() throws MessagingException {
+        return generateResponse(Session.getDefaultInstance(new Properties()));
+    }
+
     /**
      * Generates the response email.
      * <p>