--- a/hyperkitty/models/email.py
+++ b/hyperkitty/models/email.py
@@ -24,7 +24,7 @@ import logging
 import os
 import re
 from email.message import EmailMessage
-from email.utils import formataddr
+from email.utils import formataddr, make_msgid

 from django.conf import settings
 from django.db import IntegrityError, models
@@ -175,7 +175,12 @@ class Email(models.Model):
         header_date = self.date.astimezone(tz).replace(microsecond=0)
         # Date format: http://tools.ietf.org/html/rfc5322#section-3.3
         msg["Date"] = header_date.strftime("%a, %d %b %Y %H:%M:%S %z")
-        msg["Message-ID"] = "<%s>" % self.message_id
+        try:
+            msg["Message-ID"] = "<%s>" % re.sub('[<>\s]', '',
+                                                self.message_id)
+        except:
+            msg["Message-ID"] = make_msgid()
         if self.in_reply_to:
             msg["In-Reply-To"] = unfold(self.in_reply_to)