If you get "Sending of message failed...Check your 'temporary
directory' setting." error when only sending some files (but not all),
double-check that there are no special characters in the path to that
particular file.

I went through many of the troubleshooting steps in previous threads,
but came up empty, until I noticed that there was a very tiny special
character at the end of one of the folder names. In thunderbird it
looked like "_", but if a document in that folder was opened in
Microsoft Word (for instance), it looked something like "[]"

I removed the special character, and tested it. Then, everything
worked like a charm. No one knows how that special character came to
be, but I hope this helps someone in the future if they are
experiencing the same problem.