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Pretty good idea do do this way when a new mail shall be written and no account is preselected. But when I don`t want to send it but to store it only in "draft/Entwürfe" and no account is selected, it seems to store but it doesn't (because the program doesn't know where to store?). No error message pops up and the the mail is not stored and the text is lost, because the editor closes then! PLEASE CORRECT THIS!

Это отзыв для предыдущей версии этого дополнения (1.1). 

Another user recently pointed out the exact same thing to me.
I think your assumption is correct in that the draft save fails because TB doesn't know where to store it when no sender account is selected. That would make sense.
I'm currently trying out different solutions to the problem to try to find the best one.
One idea is to add yet another option which would have NoComposeAccount force you to choose a sender mail account when beginning a new message.
Hopefully a new version will be ready soon.

*edit* fix was added in 1.2