Index: Lib/email/utils.py
===================================================================
--- Lib/email/utils.py	(revision 76258)
+++ Lib/email/utils.py	(working copy)
@@ -172,10 +172,22 @@
 
 
 
+try:
+    from threading import Lock
+except ImportError:
+    from dummy_threading import Lock
+_seq_lock = Lock()
+
+def _gen_seq():
+    while True:
+        for number in xrange(100000):
+            yield number
+_seq = _gen_seq()
+
 def make_msgid(idstring=None):
     """Returns a string suitable for RFC 2822 compliant Message-ID, e.g:
 
-    <20020201195627.33539.96671@nightshade.la.mastaler.com>
+    <20020201195627.137.33539.96671@nightshade.la.mastaler.com>
 
     Optional idstring if given is a string used to strengthen the
     uniqueness of the message id.
@@ -184,12 +196,14 @@
     utcdate = time.strftime('%Y%m%d%H%M%S', time.gmtime(timeval))
     pid = os.getpid()
     randint = random.randrange(100000)
+    with _seq_lock:
+        seqint = next(_seq)
     if idstring is None:
         idstring = ''
     else:
         idstring = '.' + idstring
     idhost = socket.getfqdn()
-    msgid = '<%s.%s.%s%s@%s>' % (utcdate, pid, randint, idstring, idhost)
+    msgid = '<%s.%s.%s.%s%s@%s>' % (utcdate, pid, randint, seqint, idstring, idhost)
     return msgid
 
 
Index: Lib/email/test/test_email.py
===================================================================
--- Lib/email/test/test_email.py	(revision 76291)
+++ Lib/email/test/test_email.py	(working copy)
@@ -10,6 +10,8 @@
 import unittest
 import warnings
 from cStringIO import StringIO
+try: from threading import Thread
+except ImportError: from dummy_threading import Thread
 
 import email
 
@@ -2270,6 +2272,30 @@
         addrs = Utils.getaddresses(['User ((nested comment)) <foo@bar.com>'])
         eq(addrs[0][1], 'foo@bar.com')
 
+    def test_utils_make_msgid(self):
+        # Test make_msgid uniqueness, even with multiple threads
+
+        class MsgidsThread(Thread):
+            def run(self):
+                # generate msgids for 3 seconds
+                self.msgids = []
+                append = self.msgids.append
+                make_msgid = Utils.make_msgid
+                clock = time.clock
+                
+                tfin = clock() + 3.0
+                while clock() < tfin:
+                    append(make_msgid())
+            
+        eq = self.assertEqual
+        threads = [MsgidsThread() for i in range(5)]
+        for t in threads:
+            t.start()
+        for t in threads:
+            t.join()
+        all_ids = sum([t.msgids for t in threads], [])
+        eq(len(set(all_ids)), len(all_ids))
+
     def test_utils_quote_unquote(self):
         eq = self.assertEqual
         msg = Message()