Mail stay into send box

If for unknown reason sending a message does not work, the mail stay in the send box, eM does try to send ir again, no alert is given to say the mail is not send and I found no way to resend it

If the email is in the Sent folder, then it has been sent.

If you want to send it again, copy it to the Outbox. It will stay there until the next sync, then be delivered.