diff --git a/src/collective/dms/mailcontent/dmsmail.py b/src/collective/dms/mailcontent/dmsmail.py
index 0a2ad4d..00d33cb 100644
--- a/src/collective/dms/mailcontent/dmsmail.py
+++ b/src/collective/dms/mailcontent/dmsmail.py
@@ -156,6 +156,16 @@ class DmsIncomingMail(DmsDocument):
 @grok.subscribe(IDmsIncomingMail, IObjectAddedEvent)
 def incrementIncomingMailNumber(incomingmail, event):
     """ Increment the value in registry """
+    # if internal_reference_no is empty, we force the value.
+    # useless if the internal_reference_no field is hidden (in this case,
+    #                                                       default value must be empty to bypass validator)
+    # useless to manage automatically the internal_reference_no value without user action
+    if not incomingmail.internal_reference_no:
+        internal_reference_no = evaluateInternalReference(incomingmail, incomingmail.REQUEST,
+                              'collective.dms.mailcontent.browser.settings.IDmsMailConfig.incomingmail_number',
+                              'collective.dms.mailcontent.browser.settings.IDmsMailConfig.incomingmail_talexpression')
+        incomingmail.internal_reference_no = internal_reference_no
+        incomingmail.reindexObject(idxs=('Title', internal_reference_no))
     registry = getUtility(IRegistry)
     registry['collective.dms.mailcontent.browser.settings.IDmsMailConfig.incomingmail_number'] += 1
 
diff --git a/src/collective/dms/mailcontent/tests/test_content.py b/src/collective/dms/mailcontent/tests/test_content.py
index 7d8bf34..0fba443 100644
--- a/src/collective/dms/mailcontent/tests/test_content.py
+++ b/src/collective/dms/mailcontent/tests/test_content.py
@@ -66,3 +66,15 @@ class TestDmsmailMethods(TestContentTypes):
                           'collective.dms.mailcontent.browser.settings.IDmsMailConfig.incomingmail_number',
                           'collective.dms.mailcontent.browser.settings.IDmsMailConfig.incomingmail_talexpression'),
                           'test-in/10')
+
+    def test_incrementIncomingMailNumber(self):
+        registry = getUtility(IRegistry)
+        old_value = registry['collective.dms.mailcontent.browser.settings.IDmsMailConfig.incomingmail_number']
+        imail1 = createContentInContainer(self.portal, 'dmsincomingmail', **{'internal_reference_no': '12345',
+                                                                             'title': 'Test 1'})
+        self.assertEquals(registry['collective.dms.mailcontent.browser.settings.IDmsMailConfig.incomingmail_number'],
+                          old_value+1)
+        self.assertEquals(imail1.internal_reference_no, '12345')
+        # we create a dmsincomingmail without internal_reference_no, it will be generated
+        imail2 = createContentInContainer(self.portal, 'dmsincomingmail', **{'title': 'Test 2'})
+        self.assertEquals(imail2.internal_reference_no, 'test-in/11')