A user reported an issue with attaching files, and I was able to reproduce what I believe they experienced. When you attach a file, and mistakenly try to attach the same file again you get the following warning message -

 

"Cannot attach the same file more than once"

Now, if you were to try and send this email it prevents you from doing so, and gives the following error -

 

 

The only way to send this email would be to close the compose window, and then compose the email again. It seems as if the warning message stating you "Cannot attach the same file twice" is not canceling out the upload process of the attachment so when you go to send the email SmarterMail is still trying to upload the file and causes the error.