Do you compose in plain text mode or in HTML? Do you set Thunderbird to send both? Correspondents using text-only email clients may not see HTML-formatted content. Or, depending on how you create an HTML posting, such as pasting from somewhere else, you might be sending messages that don't fully comply with the relevant standards and are therefore not rendered by over-particular email clients. Do the correspondents who recieve empty messages have any common factor, such as the email client they are using? I've seen several broken messages originated by Apple email products.