Thunderbird Sent items shows wrong message

Yesterday I sent an email, with one attachment, to another company. The title etc of that message can be seen among the listing of Sent items, although the icon of the attachment is not shown in the listing. When I open that sent message, a totally different sent message I sent one hour later is shown. When I look the source of that sent item, it is full of content from several other messages, 862 KB to be exact. But the message which I sent is not there. What happened? The company I sent the original message today replied and in their reply my message was shown, so my message was sent correctly.

Thanks for your advice, Hannu Saarenmaa, Finland