Sending of message failed an error occurred while sending mail
Trying to send mail. Getting a message Sending of message failed An error occurred while sending mail. Relaying denied you must check for new mail before sending
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Looks like your outgoing (SMTP) server uses a form of authentication known as "POP before SMTP", where the mail client (Thunderbird) has to authenticate with the incoming (POP) server first in order to be able to use the SMTP server. The POP authentication remains valid for a few minutes.
You may be able to solve the problem by checking for new mail more frequently (e.g. every 5 mins, maybe 10), or manually checking for mail right before sending.