They send it to everyone.

You scared me with the $18000 thing; I'm a bit over that and would hate to owe back taxes so I looked it up. Turns out it's pretty complicated. Your SSDI itself really isn't taxable but if you earn more than an amount beyond that which brings you to a "base value" set each year then you have to pay taxes.

This explains why last year I tried desperately to file to explain a small amount I earned online and couldn't get the programs to accept that it was taxable, take my money! It wasn't really taxable...oh.