IANAL but…to all the people saying "Ignore the letter, don't respond until they issue a suit" - the patent holder has notified you that they believe you're infr

IANAL but…to all the people saying "Ignore the letter, don't respond until they issue a suit" - the patent holder has notified you that they believe you're infringing. If you ignore them, your actions might then be seen to be "willful infringement" of the patent, and a court can increase damages based on that. Personally, I'd probably ignore them too - but I'd double-check with a lawyer first!