All forms of signing email are generally solving the wrong problem (a thesis)
Modern email is full of forms of signed email. Personally signed email is the old fashioned approach (and wrong), but modern email on the Internet is laced with things like DKIM, which have the sending system sign it to identify at least who sent it. Unfortunately, the more I think about it, the more I feel that signed email is generally solving the wrong problem (and if it's solving the right one, we won't like that solution in the long run).
A while ago I wrote about why email often isn't as good as modern protocols, which is because it's what I described as an anonymous push protocol. An anonymous push protocol necessarily enables spam since it allows anyone to send you things. Describing email as 'anonymous push' makes it sound like the anonymity is the problem, which would make various forms of signing the solution (including DKIM). But this isn't really what you care about with email and requiring email to carry some strong identification doesn't solve the problem, as we've found out with all of the spam email that has perfectly good DKIM signatures for some random new domain.
(This is a version of the two sides of identity. On the Internet people can trivially have multiple identities, so while an identity is useful to only let selected people in, it's not useful to keep someone out.)
I think that what you really care about with modern communication protocols is revocable authorization. With a pull protocol, you have this directly; you tacitly revoke authorization by stopping pulling from the place you no longer like. With a push protocol, you can still require authorization that you grant, which lets you revoke that granted authorization if you wish. The closest email comes to this is having lots of customized email addresses and carefully using a different one for each service (which Apple has recently automated for iOS people).
Obviously, requiring authorization to push things to you has a fundamental conflict with any system that's designed to let arbitrary strangers contact you without prearrangement (which is the fundamental problem of spam). Modern protocols seem to deal with this in two ways (even with revocable authorization); they have some form of gatekeeping (in the form of accounts or access), and then they evolve to provide settings that let you stop or minimize the ability of arbitrary strangers to contact you (for example, Twitter's settings around who can send you Direct Messages).
(The modern user experience of things like Twitter has also evolved to somewhat minimize the impact of strangers trying to contact you; for example, the Twitter website separates new DMs from strangers from DMs from people you've already interacted with. It's possible that email clients could learn some lessons from this, for example by splitting your inbox into 'people and places you've interacted with before' and 'new contacts from strange people'. This would make DKIM signatures and other email source identification useful, apart from the bit where senders today feel free to keep changing where they're sending from.)
PS: In this view, actions like blocking or muting people on Twitter (or the social network of your choice) is a form of revoking their tacit authorization to push things to you.
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