The error message in the title shows up repeatedly int the mailman-web logs. Looking at the offending email with `mailman qfile`, it appears this is a legitimate email sent from an Android phone. Please tune Hyperkitty so it would be less strict regarding emails acceptance. I can provide the pickle file via private email if needed. Note: issue previously discussed on the mailman-users list: https://lists.mailman3.org/archives/list/mailman-users@mailman3.org/thread/OBD26AWULYQJE6FVDW2TOXGIKXGCZOIT/#OBD26AWULYQJE6FVDW2TOXGIKXGCZOIT