--- old/src/java.base/share/classes/sun/net/smtp/SmtpProtocolException.java	2019-08-27 17:29:30.538324386 -0700
+++ new/src/java.base/share/classes/sun/net/smtp/SmtpProtocolException.java	2019-08-27 17:29:30.322324386 -0700
@@ -32,6 +32,7 @@
  * an SMTP session.
  */
 public class SmtpProtocolException extends IOException {
+    @java.io.Serial
     private static final long serialVersionUID = -7547136771133814908L;
 
     SmtpProtocolException(String s) {