plain text e-mails without charset do not work
in fact, an exception is thrown in apps/sources/models.py
if content_type == 'text/plain' and source.store_body: content = part.get_payload(decode=True).decode(part.get_content_charset())
because part.get_content_charset() is None.
I suggest using "ASCII" or "iso-8859-15" instead of None, better yet would be something like ascii that just ignores characters with 8th bit set or replaces it with something like #XX, XX being the hex code of the offending character.