Politicians, particularly Congressmen, tell me that the mass form-letter emails they get, from unknown parties, aren't useful.
What they want are emails from constituents, preferably not form letters, but that might be okay.
Well, looks like that might already be available at a newish site, You2Gov.com:
At You2Gov.com, registered users can log on and find 1) up to the minute news stories and clips; 2) Forums where political discussions take place; and 3) a tool that allows you to call, email or write your elected officials with just one click. Senators and Congressman benefit too. You2Gov requires its users to provide their name, mailing address and zip code on communications to Congress so it can be verified that the writers are actually their constituents. Unless such proper user identification is provided, You2Gov will not allow the communications to be sent. Better yet, for those elected officials, they won't be receiving thousands of "form" letters. The You2Gov generated letters come from registered users that took the time to log on, research the issues, and write their own letter.
Note the need for authentication; this form doesn't prove the sender is a constituent, but moves pretty far in that direction.