CenterPoint is an online video magazine for online teachers. Each quarter CenterPoint publishes a new video covering topics of interest to faculty teaching online. This episode includes "Saving Time in the Online Class." They suggest collecting the emails from your students for a semester, then compiling them into an FAQ to post the next semester to save you the time it takes to answer the same questions over and over.
Of course we all know, getting the students to read the FAQ is another topic altogether! You may want to incorporate the information into your syllabus or into the assignment that generates the question.
Here's the link to watch the CenterPoint video in case the link above doesn't work: