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As you can see, each failed message is highlighted in light reddish color. Possibility to re-send message is included. Option for doing so, (known as "try again") is available at at the bottom of each failed to be be send message (2).
Successfully re-sent email is marked with a white background and placed by default at the bottom (as being the newest) in a current thread:
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For more information how to change order of the received and sent messages check One thread per new email [2.4.x]