mailto: not responding in .epub export

Having produced a book in InDesign with specific links jumping between multiple files successfully I find myself with one final issue that I am failing to get to the bottom of. I have 2 email addresses active as buttons that when exported to interactive PDF work perfectly well, but having now switched to .epub export, these have stopped responding when clicked. They remain active for rollovers and if switched to a URL link, work as expected, but the mailto: commend is completely failing. I have tried as both hyperlinks and as button actions but neither are giving any success.

I have a very limited knowledge of code so finding a fix within Indesign would be a massive help if anyone has any suggestions?