Within an organisation in principle S/MIME can provide that value, on the wider Internet there's never been a serious attempt to do so. What exists already is left as it is, basically to rot.
Things you'd want (off the top of my head, there are probably more)
- One or more agreed mechanisms to ensure the legitimate owner of name@example.com can get a cert for name@example.com but no-one else can. If the owners of example.com can, you need a rationale for why that's OK, which will be a struggle - consider whether you think tialaramex@gmail.com is me, or might also be just anybody who works for Google...
- Oversight to ensure that the rules get followed. This is truly difficult to achieve, I think we got there for the Web PKI, but it's a journey not a destination and every year is a further struggle. Interest in doing this for S/MIME tends to fizzle after just a few weeks each time.
- Client interop good enough that any normal user can expect to send S/MIME signed and encrypted mail to any other normal user and have it be delivered
- Clients that can go get a user their certificate for myname@example.com using a key they created, not some centrally administrated key which renders this whole process useless outside of a centralised trust environment.
- Widespread clients that provide a reliable UX in respect of these features to ordinary end users.
Don't hold your breath.