Clarified documentation

Author: bbottema

modules/simple-java-mail/src/main/java/org/simplejavamail/email/internal/InternalEmail.java

@@ -25,7 +25,8 @@ public InternalEmail(@NotNull EmailPopulatingBuilder builder) {
 
     /**
      * @deprecated Don't use this method. This method is used internally to set the reference to the original email when a copy is made to which all defaults and overrides
-     * are applied. When sending the email, however, we still need a reference to the original email to be able to update the message id.
+     * are applied. When sending the email, however, we still need a reference to the original email to be able to update the message id. userProvidedEmail can be set to
+     * null in some junit tests.
      */
     public void setUserProvidedEmail(@NotNull final Email userProvidedEmail) {
         this.userProvidedEmail = (InternalEmail) userProvidedEmail;
@@ -50,4 +51,5 @@ public void updateId(@NotNull final String id) {
     public boolean wasMergedWithSmimeSignedMessage() {
         return wasMergedWithSmimeSignedMessage;
     }
+
 }

modules/simple-java-mail/src/main/java/org/simplejavamail/mailer/internal/EmailGovernanceImpl.java

@@ -232,6 +232,8 @@ public EmailWithDefaultsAndOverridesApplied produceEmailApplyingDefaultsAndOverr
 		builder.fixingMessageId(resolveEmailProperty(provided, EmailProperty.ID));
 
 		val email = builder.buildEmail();
+
+		// we need to update the user's email instance with the generated ID when sending
 		if (provided != null) {
 			//noinspection deprecation
 			((InternalEmail) email).setUserProvidedEmail(provided);