Hi keren,
Based on your description, my understanding is when you select the shape of employee, you send an email to the employee with fixed content. Or you just want to open the email and with the email address of employee, and you can custom content.
For the first situation, there are no click events in Visio, its hard to get your result even use Macro code.
For the second situation, you can add a hyperlink to the shape manually and in the address box, type
mailto:email address, for example:
mailto:sample@microsoft.com
But if your employees are huge numbers, I suggest you post your issue to MSDN forum to ask for helping.
https://social.msdn.microsoft.com/Forums/en-US/home?category=visio2010&filter=alltypes&sort=lastpostdesc
Maybe you can get your result by using code.
Hope its helpful.
Regards,
George Zhao
TechNet Community Support