One of the hardest things I ever did was walk away from an over $300k licensing deal with a major corporation. I was selling them my polymail email classifier to improve their anti-spam product. They had already done all the work and proved that it was way better than their existing anti-spam component for their personal security suite.

But they were unwilling to pay a reasonable price and I wasn't willing to be screwed.

Their product's anti-spam still sucks.