If you serve a geographic district, WP Issues CRM can often determine whether email comes from your constituents.  
All in one string in any order, enter cities, zip codes or two letter abbreviations for states that you wish to consider as comprising your constituency.  
For example, "NH|Belmont|Watertown|02134|02135|02116".  Use this setting together with the Non-constituent Reply setting or just use it to update your database as email contacts are automatically logged.  
For existing constituents with already known constituency status, the saved status will override the results of the Constituents logic.  Message lines containing messages from known constituents will be highlighted blue in the inbox. 