Give the calendar a descriptive name such as Book Meeting Rooms.From the calendar settings choose "Create column" under the Columns options.
The default behavior of Share Point is to redirect the user to the url defined by the // Will abort the thread _current Context.
If the value equals what you want then run the worlflow. If the email is being received with any modification then you have got your flow wrong.
Martin's solution is very straightforward but you can also expand on this by using content types to allow for an approval process that performs another action.
Edit: For good looking errors you can redirect user to custom error page (which you have to create) I don't think there is a way to go back to edit form and see changed values.
This is my first post in 2013, here I just want to discuss about Event Receivers in Share Point 2010 (SPItem Event Receiver class) and an important factor with the Event Receivers.