First (local) working version

offlineimap.conf

@@ -1054,6 +1054,24 @@ remoteuser = username
 #reference = Mail
 
 
+
+# This option stands in the [Repository RemoteExapmle] section.
+#
+# Some mail provider like https://www.fastmail.com/ display in their web interface
+# the internaldate of their emails. Therefore it is sometimes desirable, to set the 
+# internaldate to the  value of the Date header.
+#
+# Available options are: 
+#   - timestamp (the timestamp of the local file)
+#   - header (the Date header of the email)
+#   - imap (let the IMAP server set the internaldate)
+#
+# Default is timestamp
+#
+#internal_date_for_append_method = header  
+
+
+
 # DEPRECATED: This option stands in the [Repository RemoteExample] section.
 #
 # IMAP defines an encoding for non-ASCII ("international") characters. Enable

offlineimap/folder/IMAP.py

@@ -534,11 +534,16 @@ class IMAPFolder(BaseFolder):
         :returns: string in the form of "DD-Mmm-YYYY HH:MM:SS +HHMM"
                   (including double quotes) or `None` in case of failure
                   (which is fine as value for append)."""
-
-        if rtime is None:
+        
+        # if config parameter is set respect this setting:
+        # for the default value we don't need to change anything
+        internal_date_for_append_method = self.repository.getconf('internal_date_for_append_method', 'timestamp')
+        
+        if internal_date_for_append_method == 'imap':
+            return None
+        elif internal_date_for_append_method == 'header':
             rtime = emailutil.get_message_date(content)
-            if rtime == None:
-                return None
+
         datetuple = time.localtime(rtime)
 
         try: