National Security Letters can't compel engineers/companies to do pretty much anything. National Security Letters can only request non-content information. A NSL

National Security Letters can't compel engineers/companies to do pretty much anything. National Security Letters can only request non-content information. A NSL also can't compel the gathering of additional non-content information beyond that already being gathered. So a NSL could request the records for who you are sending encrypted messages to but not the content of the messages and if your messaging provider can't or doesn't already gather that information they can't be compelled to start gathering such information. If a NSL requests information which the recipient believes violates that guidelines of information which can be requested they can disclose the NSL to legal counsel and challenge the NSL in court.