Unable to append a message to a folder: The message has been rejected by the IMAP server.

Possible causes:

  1. One or more messages have an empty subject line or a line that has a subject that has only a “Re:”
  2. The message has a malformed or corrupted header 


To solve it you need to delete or move the offending messages to a different folder.

Additional Information:

  • https://github.com/rgrove/larch/issues/24