src/java.base/share/classes/sun/net/smtp/SmtpProtocolException.java

*** 30,39 ****
--- 30,40 ----
  /**
   * This exception is thrown when unexpected results are returned during
   * an SMTP session.
   */
  public class SmtpProtocolException extends IOException {
+     @java.io.Serial
      private static final long serialVersionUID = -7547136771133814908L;
  
      SmtpProtocolException(String s) {
          super(s);
      }