In most cases receipt of an e-mailed subpoena could be shown to have been opened by looking at a mail provider's logs (ex Gmail). It's likely that a core developer for the Tor Project does not receive email in such a way, meaning receipt could be trivialy denied.

But this is all beside the point, because an NSL could never be sent to anyone over internet postcard. Clinton discussed national secrets over email and look what happened to her.

They absolutely affect the validity if people claim non-receipt. Subpoenaing cooperative people is easy - you use mail, or email, or whatever you want and they acknowledge the thing and respond. That's why email is common.

The question is how things go when someone doesn't feel like playing ball. With email and not-signed-for letters there's no reasonable way to prove that the person saw the content. "Spam probably ate it." "It must have gotten lost in the mail." And so on.

So hand-served (and signed letter) subpoenas remain relevant for when people are dodging you. The fact that many people do respond to email subpoenas doesn't relate to whether non-respondents can be charged for their failure.