Hi Kostya,
To define default account we have a lot of oprions, last used, one from the list, I would propose another option,
when we open a new message, a popup with the list appears, to select the account to use in that time.
It is not the same to open the last or one signed as defaul, because, for example, I have work email as default, but in some case I send messages from this account and I lost to modify the account before to send, so the result is a wrong account used....
thanks