Reading the letter for what it really asks
Some letters come from competitors that make a product of their own, and others from companies whose business is licensing patents. The difference affects what a realistic resolution looks like. Check whether the patents are still in force, who the recorded owner is, and whether the letter identifies specific claims and products or just gestures at an industry. Many states have laws aimed at bad-faith patent demand letters, and a vague, threatening letter sent to many small businesses can raise questions under them. Do not reply directly or say anything about your product before speaking with counsel, because early statements can be quoted later.
Why the letter itself changes your position
Receiving notice of a patent can matter later. It may be used to argue that continued sales were knowing, which bears on whether a court considers increased damages for willful infringement, and it can affect when damages start to run for patents whose owners did not mark their products. A considered opinion from patent counsel on non-infringement or invalidity is one way companies document good-faith reasoning, though whether to obtain one is a strategic choice rather than a requirement. Preserve the letter, the envelope or email headers, and internal discussion about the product from that point on, and pause any routine deletion that might reach them.
Options on both sides of the letter
Responses range from a request for the claim charts the sender is relying on, to a detailed non-infringement position, to licensing talks, to a validity challenge at the USPTO. A recipient facing a real controversy may in some circumstances file its own declaratory judgment action, which is also a risk senders should weigh before mailing a letter with sharp language. If you are the one sending, a letter that identifies the patent and the product clearly while avoiding threats against customers usually holds up better. In a first meeting we review the patents, the product, the sender's history, and any supplier indemnity that might shift the cost of responding.
