I don't believe there is a way, and I don't think that it should. It should only be bold if there is something in it that is ready to send as it displays now.
This keeps it consistent with the other folders where the folder only displays bold if there is a message that has not been read. The Outbox only displays bold if there is a message that has not been sent, and not just if it contains a message. Folders that contain messages that are read do not display bold.
I understand. I'm a busy person and always running in a lot of directions. It would be nice to be reminded that I have a draft waiting for me to be completed and sent to a recipient. I saw the feature in Thunderbird (and I'm sure it's in other mail clients), and liked it a lot.
-- -MikeD
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