I had an issue where I sent a message and 5 days later I was wondering why I didn't get a reply and then I went and searched my sent message folder and found that it had been queued. For 5 days!
Every other message around it was fine. This was a simple text message with no formatting, attachments, etc.
Really, if you can't send the message, you've got to surface the error to the user a lot sooner, and make it more obvious. These shadow fails are incredibly frustrating.