diff --git a/django/core/mail.py b/django/core/mail.py
index 1ac2a39908..58588bf712 100644
--- a/django/core/mail.py
+++ b/django/core/mail.py
@@ -247,8 +247,14 @@ class EmailMessage(object):
         msg['Subject'] = self.subject
         msg['From'] = self.from_email
         msg['To'] = ', '.join(self.to)
-        msg['Date'] = formatdate()
-        msg['Message-ID'] = make_msgid()
+
+        # Email header names are case-insensitive (RFC 2045), so we have to
+        # accommodate that when doing comparisons.
+        header_names = [key.lower() for key in self.extra_headers]
+        if 'date' not in header_names:
+            msg['Date'] = formatdate()
+        if 'message-id' not in header_names:
+            msg['Message-ID'] = make_msgid()
         for name, value in self.extra_headers.items():
             msg[name] = value
         return msg
diff --git a/tests/regressiontests/mail/tests.py b/tests/regressiontests/mail/tests.py
index c8b85f48c6..c2c084c4c6 100644
--- a/tests/regressiontests/mail/tests.py
+++ b/tests/regressiontests/mail/tests.py
@@ -52,4 +52,12 @@ BadHeaderError: Header values can't contain newlines (got u'Subject\nInjection T
 >>> message.as_string()
 'Content-Type: text/plain; charset="utf-8"\nMIME-Version: 1.0\nContent-Transfer-Encoding: quoted-printable\nSubject: Long subject lines that get wrapped should use a space continuation\n character to get expected behaviour in Outlook and Thunderbird\nFrom: from@example.com\nTo: to@example.com\nDate: ...\nMessage-ID: <...>\n\nContent'
 
+# Specifying dates or message-ids in the extra headers overrides the defaul
+# values (#9233).
+
+>>> headers = {"date": "Fri, 09 Nov 2001 01:08:47 -0000", "Message-ID": "foo"}
+>>> email = EmailMessage('subject', 'content', 'from@example.com', ['to@example.com'], headers=headers)
+>>> email.message().as_string()
+'Content-Type: text/plain; charset="utf-8"\nMIME-Version: 1.0\nContent-Transfer-Encoding: quoted-printable\nSubject: subject\nFrom: from@example.com\nTo: to@example.com\ndate: Fri, 09 Nov 2001 01:08:47 -0000\nMessage-ID: foo\n\ncontent'
+
 """