Fix for #79 that adds support for custom ID value. Closes #77.

src/main/java/org/simplejavamail/converter/internal/mimemessage/MimeMessageHelper.java

@@ -7,6 +7,7 @@
 import org.simplejavamail.email.AttachmentResource;
 import org.simplejavamail.email.Email;
 import org.simplejavamail.email.Recipient;
+import org.simplejavamail.internal.util.MiscUtil;
 
 import javax.activation.DataHandler;
 import javax.activation.DataSource;
@@ -64,7 +65,16 @@ public static MimeMessage produceMimeMessage(final Email email, final Session se
 		}
 		// create new wrapper for each mail being sent (enable sending multiple emails with one mailer)
 		final MimeEmailMessageWrapper messageRoot = new MimeEmailMessageWrapper();
-		final MimeMessage message = new MimeMessage(session);
+		final MimeMessage message = new MimeMessage(session) {
+			@Override
+			protected void updateMessageID() throws MessagingException {
+				if (valueNullOrEmpty(email.getId())) {
+					super.updateMessageID();
+				} else {
+					setHeader("Message-ID", email.getId());
+				}
+			}
+		};
 		// set basic email properties
 		message.setSubject(email.getSubject(), CHARACTER_ENCODING);
 		message.setFrom(new InternetAddress(email.getFromRecipient().getAddress(), email.getFromRecipient().getName(), CHARACTER_ENCODING));

src/main/java/org/simplejavamail/email/Email.java

@@ -39,6 +39,16 @@
  */
 @SuppressWarnings("SameParameterValue")
 public class Email {
+
+	/**
+	 * Optional ID, which will be used when sending using the underlying Java Mail framework. Will be generated otherwise.
+	 * <p>
+	 * Note that id can only ever be filled by end-users for sending an email. This library will never fill this field when converting a MimeMessage.
+	 * <p>
+	 * The id-format should be conform <a href="https://tools.ietf.org/html/rfc5322#section-3.6.4">rfc5322#section-3.6.4</a>
+	 */
+	private String id;
+
 	/**
 	 * The sender of the email. Can be used in conjunction with {@link #replyToRecipient}.
 	 */
@@ -173,6 +183,13 @@ public void signWithDomainKey(@Nonnull final InputStream dkimPrivateKeyInputStre
 		this.selector = checkNonEmptyArgument(selector, "selector");
 	}
 
+	/**
+	 * Bean setter for {@link #id}.
+	 */
+	public void setId(@Nullable final String id) {
+		this.id = id;
+	}
+
 	/**
 	 * Sets the sender address.
 	 *
@@ -355,6 +372,13 @@ public void addAttachment(@Nullable final String name, @Nonnull final DataSource
 		attachments.add(new AttachmentResource(MiscUtil.encodeText(name), filedata));
 	}
 
+	/**
+	 * Bean getter for {@link #id}.
+	 */
+	public String getId() {
+		return id;
+	}
+
 	/**
 	 * Bean getter for {@link #fromRecipient}.
 	 */
@@ -452,6 +476,7 @@ public boolean equals(final Object o) {
 	@Override
 	public String toString() {
 		return "Email{" +
+				"\n\tid=" + id +
 				"\n\tfromRecipient=" + fromRecipient +
 				",\n\treplyToRecipient=" + replyToRecipient +
 				",\n\ttext='" + text + '\'' +
@@ -476,6 +501,7 @@ public String toString() {
 		attachments = builder.getAttachments();
 		headers = builder.getHeaders();
 
+		id = builder.getId();
 		fromRecipient = builder.getFromRecipient();
 		replyToRecipient = builder.getReplyToRecipient();
 		text = builder.getText();

src/main/java/org/simplejavamail/email/EmailBuilder.java

@@ -40,6 +40,16 @@
  */
 @SuppressWarnings("UnusedReturnValue")
 public class EmailBuilder {
+
+	/**
+	 * Optional ID, which will be used when sending using the underlying Java Mail framework. Will be generated otherwise.
+	 * <p>
+	 * Note that id can only ever be filled by end-users for sending an email. This library will never fill this field when converting a MimeMessage.
+	 * <p>
+	 * The id-format should be conform <a href="https://tools.ietf.org/html/rfc5322#section-3.6.4">rfc5322#section-3.6.4</a>
+	 */
+	private String id;
+
 	private Recipient fromRecipient;
 	/**
 	 * The reply-to-address, optional. Can be used in conjunction with {@link #fromRecipient}.
@@ -139,13 +149,18 @@ public EmailBuilder() {
 		}
 	}
 
-	/**
-	 *
-	 */
 	public Email build() {
 		return new Email(this);
 	}
 
+	/**
+	 * Sets the optional id to be used when sending using the underlying Java Mail framework. Will be generated otherwise.
+	 */
+	public EmailBuilder id(@Nullable final String id) {
+		this.id = id;
+		return this;
+	}
+
 	/**
 	 * Sets the sender address {@link #fromRecipient}.
 	 *
@@ -523,6 +538,10 @@ public EmailBuilder signWithDomainKey(@Nonnull final InputStream dkimPrivateKeyI
 		SETTERS / GETTERS
 	 */
 
+	public String getId() {
+		return id;
+	}
+
 	public Recipient getFromRecipient() {
 		return fromRecipient;
 	}

src/test/java/org/simplejavamail/mailer/MailerLiveTest.java

@@ -1,6 +1,7 @@
 package org.simplejavamail.mailer;
 
 import org.junit.Before;
+import org.junit.Ignore;
 import org.junit.Rule;
 import org.junit.Test;
 import org.simplejavamail.email.AttachmentResource;
@@ -14,12 +15,14 @@
 import testutil.testrules.TestSmtpServer;
 
 import javax.mail.MessagingException;
+import javax.mail.internet.MimeMessage;
 import java.io.IOException;
 import java.util.Properties;
 
 import static javax.mail.Message.RecipientType.TO;
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.simplejavamail.converter.EmailConverter.mimeMessageToEmail;
+import static org.simplejavamail.internal.util.MiscUtil.valueNullOrEmpty;
 import static testutil.EmailHelper.normalizeText;
 import static testutil.EmailHelper.readOutlookMessage;
 
@@ -45,6 +48,13 @@ public void createMailSession_StandardDummyMail()
 		assertSendingEmail(EmailHelper.createDummyEmail());
 	}
 
+	@Test
+	@Ignore("Unfortunately, Wiser doesn't seem to get the ID back, but I confirmed with gmail that the (correct) ID should be there")
+	public void createMailSession_StandardDummyMailWithId()
+			throws IOException, MessagingException {
+		assertSendingEmail(EmailHelper.createDummyEmail("<123@456>"));
+	}
+
 	@Test
 	public void createMailSession_OutlookMessageTest()
 			throws IOException, MessagingException {
@@ -78,7 +88,11 @@ public void createMailSession_OutlookMessageTest()
 	private Email assertSendingEmail(final Email originalEmail)
 			throws MessagingException {
 		mailer.sendMail(originalEmail);
-		Email receivedEmail = mimeMessageToEmail(smtpServerRule.getOnlyMessage());
+		MimeMessage receivedMimeMessage = smtpServerRule.getOnlyMessage();
+		if (!valueNullOrEmpty(originalEmail.getId())) {
+			 assertThat(receivedMimeMessage.getMessageID()).isEqualTo(originalEmail.getId());
+		}
+		Email receivedEmail = mimeMessageToEmail(receivedMimeMessage);
 		assertThat(receivedEmail).isEqualTo(originalEmail);
 		return receivedEmail;
 	}

src/test/java/testutil/EmailHelper.java

@@ -2,6 +2,7 @@
 
 import org.simplejavamail.email.Email;
 
+import javax.annotation.Nullable;
 import javax.mail.util.ByteArrayDataSource;
 import java.io.IOException;
 import java.io.InputStream;
@@ -12,10 +13,15 @@
 import static org.simplejavamail.converter.EmailConverter.outlookMsgToEmail;
 
 public class EmailHelper {
-
-	public static Email createDummyEmail()
+
+	public static Email createDummyEmail() throws IOException {
+		return createDummyEmail(null);
+	}
+
+	public static Email createDummyEmail(@Nullable String id)
 			throws IOException {
 		final Email emailNormal = new Email();
+		emailNormal.setId(id);
 		emailNormal.setFromAddress("lollypop", "[email protected]");
 		// normally not needed, but for the test it is because the MimeMessage will
 		// have it added automatically as well, so the parsed Email will also have it then
@@ -25,7 +31,7 @@ public static Email createDummyEmail()
 		emailNormal.setText("We should meet up!");
 		emailNormal.setTextHTML("<b>We should meet up!</b><img src='cid:thumbsup'>");
 		emailNormal.setSubject("hey");
-
+		
 		// add two text files in different ways and a black thumbs up embedded image ->
 		ByteArrayDataSource namedAttachment = new ByteArrayDataSource("Black Tie Optional", "text/plain");
 		namedAttachment.setName("dresscode.txt"); // normally not needed, but otherwise the equals will fail
@@ -35,12 +41,12 @@ public static Email createDummyEmail()
 		emailNormal.addEmbeddedImage("thumbsup", parseBase64Binary(base64String), "image/png");
 		return emailNormal;
 	}
-
+	
 	public static Email readOutlookMessage(final String filePath) {
 		InputStream resourceAsStream = EmailHelper.class.getClassLoader().getResourceAsStream(filePath);
 		return outlookMsgToEmail(resourceAsStream);
 	}
-
+	
 	public static String normalizeText(String text) {
 		return text.replaceAll("\\r\\n", "\n").replaceAll("\\r", "\n");
 	}