When I take a long time to compose a message, including replying to a message, the send operation often fails. Because these failed messages sometimes also show up in my Drafts folder, I suspect that something in the internal state of TB interferes with the send operation because when that happens I can just discard the message version being sent unsuccessfully, go to Drafts and choose to Edit the (same) message and Send it immediately with no problem. I feel this may be a bug.