If MYOB fails to send and invoice by email and throws an I/O type error it might be because there is an Esales.pdf file already in the temp folder.
To fix simply delete all Esale.pdf files from the temp folder:
1) Open the command line from the Start menu
Start >> Run
2) Find the temp folder
This will open the temp folder from where you can delete an Esales.pdf files and MYOB should then send invoices again