diff --git a/mail/src/main/java/jakarta/mail/internet/MimeMessage.java b/mail/src/main/java/jakarta/mail/internet/MimeMessage.java
index f987ec1e..63da3ef6 100644
--- a/mail/src/main/java/jakarta/mail/internet/MimeMessage.java
+++ b/mail/src/main/java/jakarta/mail/internet/MimeMessage.java
@@ -18,7 +18,6 @@
 
 import jakarta.mail.*;
 import jakarta.activation.*;
-import java.lang.*;
 import java.io.*;
 import java.util.*;
 import java.text.ParseException;
@@ -2212,6 +2211,7 @@ public void saveChanges() throws MessagingException {
      * to override only the algorithm for choosing a Message-ID.
      *
      * @exception  	MessagingException for failures
+     * @see InternetAddress#getLocalAddress
      * @since		JavaMail 1.4
      */
     protected void updateMessageID() throws MessagingException {
diff --git a/mail/src/main/java/jakarta/mail/internet/UniqueValue.java b/mail/src/main/java/jakarta/mail/internet/UniqueValue.java
index 01a6da33..17840861 100644
--- a/mail/src/main/java/jakarta/mail/internet/UniqueValue.java
+++ b/mail/src/main/java/jakarta/mail/internet/UniqueValue.java
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1997, 2020 Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 1997, 2021 Oracle and/or its affiliates. All rights reserved.
  *
  * This program and the accompanying materials are made available under the
  * terms of the Eclipse Public License v. 2.0, which is available at
@@ -16,9 +16,10 @@
 
 package jakarta.mail.internet;
 
-import java.net.*;
+import com.sun.mail.util.PropUtil;
 import java.util.concurrent.atomic.AtomicInteger;
 import jakarta.mail.Session;
+import java.util.UUID;
 
 /**
  * This is a utility class that generates unique values. The generated
@@ -36,7 +37,7 @@ class UniqueValue {
     /**
      * A global unique number, to ensure uniqueness of generated strings.
      */
-    private static AtomicInteger id = new AtomicInteger();
+    private static final AtomicInteger id = new AtomicInteger();
 
     /**
      * Get a unique value for use in a multipart boundary string.
@@ -47,12 +48,16 @@ class UniqueValue {
      */
     public static String getUniqueBoundaryValue() {
 	StringBuilder s = new StringBuilder();
-	long hash = s.hashCode();
-
-	// Unique string is ----=_Part_<part>_<hashcode>.<currentTime>
-	s.append("----=_Part_").append(id.getAndIncrement()).append("_").
-	  append(hash).append('.').
-	  append(System.currentTimeMillis());
+	s.append("----=_Part_");
+        if (PropUtil.getBooleanSystemProperty(
+                "mail.mime.multipart.boundary.format", true)) {
+            s.append(UUID.randomUUID());
+        } else {
+            // Unique string is ----=_Part_<part>_<hashcode>.<currentTime>
+            s.append(id.getAndIncrement()).append("_").
+            append(System.identityHashCode(s)).append('.').
+            append(System.currentTimeMillis());
+        }
 	return s.toString();
     }
 
@@ -71,25 +76,45 @@ public static String getUniqueBoundaryValue() {
      * @see jakarta.mail.internet.InternetAddress
      */
     public static String getUniqueMessageIDValue(Session ssn) {
-	String suffix = null;
+	StringBuilder s = new StringBuilder();
+        if (messageIdFormat(ssn)) {
+            s.append(UUID.randomUUID());
+        } else {
+            // Unique string is <hashcode>.<id>.<currentTime><suffix>
+            s.append(System.identityHashCode(s)).append('.').
+            append(id.getAndIncrement()).append('.').
+            append(System.currentTimeMillis());
+        }
 
+        String suffix;
 	InternetAddress addr = InternetAddress.getLocalAddress(ssn);
 	if (addr != null)
 	    suffix = addr.getAddress();
 	else {
 	    suffix = "jakartamailuser@localhost"; // worst-case default
 	}
-	int at = suffix.lastIndexOf('@');
-	if (at >= 0)
-	    suffix = suffix.substring(at);
-
-	StringBuilder s = new StringBuilder();
 
-	// Unique string is <hashcode>.<id>.<currentTime><suffix>
-	s.append(s.hashCode()).append('.').
-	  append(id.getAndIncrement()).append('.').
-	  append(System.currentTimeMillis()).
-	  append(suffix);
+	int at = suffix.lastIndexOf('@');
+	if (at >= 0) {
+            s.append(suffix, at, suffix.length());
+        } else {
+            s.append(suffix);
+        }
 	return s.toString();
     }
+    
+    private static boolean messageIdFormat(Session ssn) {
+        String k = "mail.mime.messageid.format";
+        boolean def = true;
+        if (ssn != null) {
+            return PropUtil.getBooleanProperty(ssn.getProperties(), k, def);
+        } else {  //Act like default default session without creating it.
+            return PropUtil.getBooleanSystemProperty(k, def);
+        }
+    }
+    
+    // No one should instantiate this class.
+    private UniqueValue() throws IllegalAccessException {
+        throw new IllegalAccessException(UniqueValue.class.getName());
+    }
 }
diff --git a/mail/src/main/java/jakarta/mail/internet/package.html b/mail/src/main/java/jakarta/mail/internet/package.html
index d3412d9f..ba644c9c 100644
--- a/mail/src/main/java/jakarta/mail/internet/package.html
+++ b/mail/src/main/java/jakarta/mail/internet/package.html
@@ -4,7 +4,7 @@
 <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
 <!--
 
-    Copyright (c) 1997, 2020 Oracle and/or its affiliates. All rights reserved.
+    Copyright (c) 1997, 2021 Oracle and/or its affiliates. All rights reserved.
 
     This program and the accompanying materials are made available under the
     terms of the Eclipse Public License v. 2.0, which is available at
@@ -290,6 +290,19 @@
 </TD>
 </TR>
 
+<TR>
+<TD><A ID="mail.mime.messageid.format">mail.mime.messageid.format</A></TD>
+<TD>boolean</TD>
+<TD>
+Sets the format of the
+{@link jakarta.mail.internet.MimeMessage#updateMessageID message id} of the
+MimeMessage class. If set to <code>"false"</code>, the format is compatible with
+older versions of JakartaMail.  If set to <code>"true"</code> the message id
+will contain a randomly generator UUID.  The default is <code>"true"</code> if
+not specified or format is invalid.
+</TD>
+</TR>
+
 <TR>
 <TD><A ID="mail.replyallcc">mail.replyallcc</A></TD>
 <TD>boolean</TD>
@@ -344,6 +357,18 @@
 </TD>
 </TR>
 
+<TR>
+<TD><A ID="mail.mime.multipart.boundary.format">mail.mime.multipart.boundary.format</A></TD>
+<TD>boolean</TD>
+<TD>
+Sets the format of the MimeMultipart boundary line. If set to 
+<code>"false"</code>, the format is compatible with older versions of
+JakartaMail.  If set to <code>"true"</code> the boundary line will contain a
+randomly generator UUID.  The default is <code>"true"</code> if not specified or
+format is invalid.
+</TD>
+</TR>
+
 <TR>
 <TD><A ID="mail.mime.setcontenttypefilename">mail.mime.setcontenttypefilename</A></TD>
 <TD>boolean</TD>
diff --git a/mail/src/test/java/jakarta/mail/internet/UniqueValueTest.java b/mail/src/test/java/jakarta/mail/internet/UniqueValueTest.java
new file mode 100644
index 00000000..20a4f787
--- /dev/null
+++ b/mail/src/test/java/jakarta/mail/internet/UniqueValueTest.java
@@ -0,0 +1,215 @@
+/*
+ * Copyright (c) 2021 Oracle and/or its affiliates. All rights reserved.
+ *
+ * This program and the accompanying materials are made available under the
+ * terms of the Eclipse Public License v. 2.0, which is available at
+ * http://www.eclipse.org/legal/epl-2.0.
+ *
+ * This Source Code may also be made available under the following Secondary
+ * Licenses when the conditions for such availability set forth in the
+ * Eclipse Public License v. 2.0 are satisfied: GNU General Public License,
+ * version 2 with the GNU Classpath Exception, which is available at
+ * https://www.gnu.org/software/classpath/license.html.
+ *
+ * SPDX-License-Identifier: EPL-2.0 OR GPL-2.0 WITH Classpath-exception-2.0
+ */
+package jakarta.mail.internet;
+
+import java.util.Properties;
+import java.util.UUID;
+import jakarta.mail.Session;
+import org.junit.Test;
+import static org.junit.Assert.*;
+
+public class UniqueValueTest {
+    
+    private static final String MESSAGEID_KEY = "mail.mime.messageid.format";
+    
+    private static final String BOUNDARY_KEY 
+            = "mail.mime.multipart.boundary.format";
+
+    public UniqueValueTest() {
+    }
+    
+    @Test(expected = java.lang.IllegalAccessException.class)
+    public void testDeclaredConstructor() throws ReflectiveOperationException {
+        UniqueValue.class.getDeclaredConstructor().newInstance();
+    }
+
+    @Test
+    public void testDefGetUniqueBoundaryValue() {
+        String v = System.getProperty(BOUNDARY_KEY);
+        assertNull(v, v);
+        expectUuidBoundary();
+    }
+    
+    private void expectUuidBoundary() {
+        String start = "----=_Part_";
+        String result = UniqueValue.getUniqueBoundaryValue();
+        assertTrue(result, result.startsWith(start));
+        try {
+            UUID.fromString(result.substring(start.length()));
+        } catch (Throwable t) {
+            throw new RuntimeException(result, t);
+        }
+    }
+
+    @Test
+    public void testInvalidUuidGetUniqueBoundaryValue() {
+        System.setProperty(BOUNDARY_KEY, UniqueValueTest.class.getName());
+        try {
+            expectUuidBoundary();
+        } finally {
+            System.getProperties().remove(BOUNDARY_KEY);
+        }
+    }
+    
+    @Test
+    public void testUuidGetUniqueBoundaryValue() {
+        System.setProperty(BOUNDARY_KEY, "true");
+        try {
+            expectUuidBoundary();
+        } finally {
+            System.getProperties().remove(BOUNDARY_KEY);
+        }
+    }
+
+    @Test
+    public void testUvGetUniqueBoundaryValue() {
+        System.setProperty(BOUNDARY_KEY, "false");
+        try {
+            String start = "----=_Part_";
+            String result = UniqueValue.getUniqueBoundaryValue();
+            assertTrue(result, result.startsWith(start));
+            try {
+                int s = start.length();
+                int n = result.indexOf('_', s);
+                Long.parseLong(result.substring(s, n)); //id
+
+                s = n + 1;
+                n = result.indexOf('.', s);
+                Long.parseLong(result.substring(s, n)); //hashCode
+
+                Long.parseLong(result.substring(n + 1)); //millis
+            } catch (Throwable t) {
+                throw new RuntimeException(result, t);
+            }
+        } finally {
+            System.getProperties().remove(BOUNDARY_KEY);
+        }
+    }
+
+    @Test
+    public void testDefGetUniqueMessageIDValue() {
+        Properties p = new Properties();
+        String v = p.getProperty(MESSAGEID_KEY);
+        assertNull(v, v);
+        expectUuidMessageID(Session.getInstance(p));
+    }
+
+    /**
+     * If a session is given but the value is not defined ensure code will not
+     * fallback to using system properties.
+     */
+    @Test
+    public void testUuidNoheritGetUniqueMessageIDValue() {
+        System.setProperty(MESSAGEID_KEY, "false");
+        try {
+            Properties p = new Properties();
+            String v = p.getProperty(MESSAGEID_KEY);
+            assertNull(v, v);
+            expectUuidMessageID(Session.getInstance(p));
+        } finally {
+            System.getProperties().remove(MESSAGEID_KEY);
+        }
+    }
+    
+
+    @Test
+    public void testUvGetUniqueMessageIDValue() {
+        Properties p = new Properties();
+        p.put(MESSAGEID_KEY, "false");
+        String result = UniqueValue.getUniqueMessageIDValue(
+                Session.getInstance(p));
+        try {
+            int s = 0;
+            int n = result.indexOf('.', s);
+            Long.parseLong(result.substring(s, n)); //id
+
+            s = n + 1;
+            n = result.indexOf('.', s);
+            Long.parseLong(result.substring(s, n)); //hashCode
+
+            s = n + 1;
+            n = result.indexOf('@', s);
+            Long.parseLong(result.substring(s, n)); //millis
+        } catch (Throwable t) {
+            throw new RuntimeException(result, t);
+        }
+    }
+
+    @Test
+    public void testUuidGetUniqueMessageIDValue() {
+        Properties p = new Properties();
+        p.put(MESSAGEID_KEY, "true");
+        expectUuidMessageID(Session.getInstance(p));
+    }
+
+    @Test
+    public void testDefSystemGetUniqueMessageIDValue() {
+        String v = System.getProperty(MESSAGEID_KEY);
+        assertNull(v, v);
+        expectUuidMessageID((Session) null);
+    }
+    
+    @Test
+    public void testInvalidGetUniqueMessageIDValue() {
+        Properties p = new Properties();
+        p.put(MESSAGEID_KEY, UniqueValueTest.class.getName());
+        expectUuidMessageID(Session.getInstance(p));
+    }
+    
+    private void expectUuidMessageID(Session s) {
+        String result = UniqueValue.getUniqueMessageIDValue(s);
+        try {
+            UUID.fromString(result.substring(0, result.indexOf('@')));
+        } catch (Throwable t) {
+            throw new RuntimeException(result, t);
+        }
+    }
+    
+    @Test
+    public void testUuidSystemGetUniqueMessageIDValue() {        
+        System.setProperty(MESSAGEID_KEY, "true");
+        try {
+            expectUuidMessageID((Session) null);
+        } finally {
+            System.getProperties().remove(MESSAGEID_KEY);
+        }
+    }
+
+    @Test
+    public void testUvSystemGetUniqueMessageIDValue() {
+        System.setProperty(MESSAGEID_KEY, "false");
+        try {
+            String result = UniqueValue.getUniqueMessageIDValue((Session) null);
+            try {
+                int s = 0;
+                int n = result.indexOf('.', s);
+                Long.parseLong(result.substring(s, n)); //id
+
+                s = n + 1;
+                n = result.indexOf('.', s);
+                Long.parseLong(result.substring(s, n)); //hashCode
+
+                s = n + 1;
+                n = result.indexOf('@', s);
+                Long.parseLong(result.substring(s, n)); //millis
+            } catch (Throwable t) {
+                throw new RuntimeException(result, t);
+            }
+        } finally {
+            System.getProperties().remove(MESSAGEID_KEY);
+        }
+    }
+}