Thank you for your post.
FileMaker Pro uses the OS default email client.
To change the default mail application on Windows 7, Click Start and open Default Programs. Click on Set your default programs. Select the program you want to use as the default email application. Click OK.
Relaunch FileMaker Pro to read the new setting. Now, when you Send Mail, it will launch your selected email application.
Let me know if you need additional clarification.