How a Snail Mail Startup Bust Shaped One Serial EntrepreneurLearn how an engineer learned the value before building the product.

Jonathan Siegel has launched 20 companies. Two led to eight-figure exits -- hooray! -- and 16 went bust. But one failure from 2005 informed a success a decade later. The flop was PostASAP, which sent letters for users who didn't want to deal with printing, envelopes and the post office. It died in a year. Today he owns Earth Class Mail, a $5 million platform that scans, shreds or processes businesses' snail mail. Here's how he got from one to the other.

Q.What was the first sign of trouble for PostASAP?