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Does it even matter if you read or comprehend the material? IANAL. As far as I'm aware knowledge of the law does not preclude it from affecting you. It's not a contract they're giving you, they don't need your consent.

From my understanding they just notify you and it's up to you to make heads or tails of it.

I'm having trouble finding a source outside of an oblique reference in the Wikipedia article for NSLs, but yes, you can talk to an attorney about the letter. This was not the case originally (or at least in the opinion of the government), but the law was changed to allow consulting with a lawyer (and challenging the letter in court as well). IANAL, but I play one on the Internet.

I'm curious how these letters are delivered. If you know something is likely to be an NSL in the envelope, what happens if you just forward it along to someone else, such as a journalist, without ever opening it.

Do people have a legal obligation to accept and read US mail?

You probably get served with it, like a court summons or a lawsuit.