Recently, I ran into this conundrum. I was trying to send an email using the Thunderbird mail client when my connectivity was intermittent and the email got stuck in the unsent messages list. I tried resending after restarting the client, but it refused to do so. I forwarded the message to the person I was replying to just in case, I was unable to send this out. I wondered what would happen if i moved the message to the Drafts folder and Voila!!! there my message was ready to edit and I could just hit send.