Emailing a congressman, especially a Tea-bagging Repugnican, will have zero effect and won't even come to his attention. Of the thousands they must get every day, probably less than a dozen are read, and maybe one or two makes its way to the Rep's eyes.
Mailing letters to them is equally fraught. Letters now go to a processing facility where they are subjected to testing and/or irradiation to insure that they contain nothing harmful. By the time they get to the Congressman's office, they are in bundles and are treated much like the mass of emails received every day--barely glanced at.
But postcards are another story. Since they are not sealed they cannot contain harmful substances, so these tend to get directly to where they are going. Plus they are something of a novelty (I mean, who sends postcards any more?) so they will likely be read by at least one person in the office. If this is so, then the possibility increases that the message will actually get to the person intended.
I've been sending out postcards like this (below) for months, and even though Congresspersons think that they are only responsible to the people of their district, they vote on bills (or obstruct them) that affect all of us, so they are fair game for messages from ANYBODY.
In the image I've obscured my name and address at the bottom of the card; it certainly isn't required that you ID yourself, but I think it helps to let these clods know that they are stirring irritation all over the country.