Recently I experimented a copy failed error message in Thunderbird every time I tried to send an email. Why? I am not sure, but seems people is having the same problem. At GetSatisfaction we can see that Ronni had the same issue.
This may be caused by a full Sent folder, but usually happens with IMAP accounts, since this error came from an IMAP protocol message.
Roland’s instructions from that page suggested:
After following Rolands instructions, I took it one step further and fixed my issue. maybe this will work for you as well.
Right click on the sent folder in the e-mail program, scroll down to Properties, click on the General Information and make sure there is a Tick in the; Don’t delete any messages, and make sure there is NO tick on the; Always delete read messages. Test to see if this works for you. This issue actually caused me to stop using Thunderbird until todays last ditch effort to fix this issue. Happy am I that it’s working for me now!
So you can try to do that. They also suggested to follow these instructions.