You can directly put HTML tag instead of the plugin's shortcode, like this:
<p><input type="submit" name="your-submit" value="I'm Going" /></p>
<p><input type="submit" name="your-submit" value="I'm Not Going" /></p>Note that they have name attributes (name value is arbitrary and not important). Also there are no id attributes as they are useless in this case.
Then, put [your-submit] in your mail body. You can see which button has been used by checking the value.