Thank you for your post.
FileMaker Pro 12 was not tested with Mac OS X 10.10, as the last update of FileMaker Pro 12 was released well before Mac OS X 10.10 was released. For more information, see Knowledge Base Article #13951:
Regardless, on my Mac OS X 10.10.1 machine, I made sure Apple Mail was set as my default email client, and then I installed FileMaker Pro 12.0v4. After launching FileMaker Pro 12.0v4 and opening a database file, I pulled down the File menu and selected "Send -> Mail...". I addressed a message to myself and sent it. The mail was sent, and it was received at the desired address. Therefore, Send Mail should work.
Launch Mail, pull down the Mail menu and select Preferences, click the "General" icon on the top, and make sure the first entry "Default email reader" is set to Mail.
Once set, launch FileMaker Pro 12 (although the version I installed was v4, it should be fine with 12.0v5), open any database file, and try sending a dummy message.
Keep me updated with any progress.