Can different email addresses be chosen by ClickYes Pro for different MYOB data files, ie the email specific to data file 1 and another email address for data file 2?
No, you can't set emails for different data files, instead you can choose applications. I have checked the website, but there isn't anything available and related to associate various MyOB data files.
There is a page where you can see all the supported options and a sample tutorial on how to work with the application.
Official website: http://www.contextmagic.com/express-clickyes/pro-version.htm