Bug hunters found a critical flaw in Mozilla's Thunderbird 18.104.22.168 e-mail client. E-mails that don't have a unique sender-ID and message-ID and some mails send by ChillKat and Outlook Express can lead to problems.
The result is that Thunderbird stores the e-mails in your e-mail database but you won't be able to see them. The e-mails corrupt your database and once Thunderbird compresses the database it will delete these e-mails.
New beta versions feature a fix for the problem, an official version is expected in December.