If sending of messages fails because of weak network signal,
it is later impossible to send that messages by Outbox - menu - Send messages when network is good.
It is impossible even after reseting of the app (which helps for loading from imap, when it does not work).
It is impossible even at fast wifi connection.
No way how to send messages in outbox folder directly.
The fastest way is to use Outbox - concrete message - menu -
Send… - Edit as new message.
This way works, copy of the message is sent succesfully at good network signal.
But “send messages” does not work.