Earlier quoted context omitted.
I hear this, but I have absolutely zero idea how to get s/mime working and I'm faintly aware it might involve having to buy a digital certificate off someone (and doesn't that mean that someone could decrypt my email anyway? if they're the ones who generate the certificate?) What I'm trying to say here is: I'm a bit of a geek, and if I don't understand how it works, there's no way e.g. my parents are. If s/mime is th…
No, you generate your own, then have the public key signed by a CA having proven your identity to some greater or lesser degree, depending on the level of certification - but generating your own and having it signed is not a straightforward process in my (limited) experience.
The problem with that method is the recipient of your mail is still relying on that CA to validate your public key. The CA could (willingly or under duress) sign some other public key and claim it's yours, then use that key to impersonate you, and even trick recipients into using that public key to encrypt emails intended for you. That would form the basis of a man-in-the-middle attack.
It's unlikely to work if you've already been communicating using the real public key (depending on how the software handles new keys), but for new recipients in particular, it's possible.