Sometimes it still pays to use snail mail (US Postal Service in your mailbox type mail).  So many people are moving away from it, but I believe it still has its uses.

The real power of snail mail is the ability to elevate the importance of your message.  Send it Overnight or Priority Mail and it becomes more important, especially with Delivery Confirmation, so when they try to tell you they didn't receive it, you can tell exactly when it arrived and was signed for-- there is no rebuttal to that.  Want to show them you mean business?  Send it Certified Mail with a Return Receipt-- this signifies that you are carefully documenting things, because you may take them to court.

These methods are a little more expensive, but believe me, they get results.

See my post, If You Face Goliaths . . ., about dealing with people who try to push you around.