Hey there, Jon.
It appears that you're having an issue with the Mail application on your iMac. Each time you try to use the "Save as" option for a message, the Mail app stops responding and you have to force-quit the app. I'm happy to help.
macOS Sierra: Quit apps -- Make certain the Mail app is completely closed, then restart the Mail app while holding down the shift key. Test this issue at this point. If you're able to use "Save as" successfully, quit the Mail app again, restart it normally and test.
How to turn on or turn off your Mac -- If the issue persists, restart your iMac then test the issue.
Use safe mode to isolate issues with your Mac -- If restarting your iMac didn't resolve the issue, test how it responds in safe mode. If there are no issues, reboot normally and test again in your own user account. Remember this when booting in safe mode: Limited graphics performance in OS X recovery or safe mode
How to test an issue in another user account on your Mac -- If the issue is also happening in safe mode, use this article to create and test another user account. This will demonstrate if the issue is isolated to your account, or if it's system wide. You'll need to add an email account for this new user in order to test.
Let me know how that goes.