Based on my personal experience providing support for s/mime setups, you'd need:
1) A centralised solution for managing keys and certificates, connected to a login that the customer will be able to recover in pretty much any situation.
2) Email client support for fetching keys/certificates from the centralised solution.
3) A massive focus on usability and end-user support, because most email users have no idea what a certificate is, or how to use it.
Denmark actually has something similar to this (Sikker mail) but it's mainly aimed at businesses. Based on what I've seen, this resulted in a market for services that bypass the E2E aspect of it because business users can't figure out how to use it. It is also noteworthy that despite this s/mime being available for everyone, Denmark has a public digital mailbox for all citizens and businesses in order to ensure availability.
S/mime is great. It is also not suited for people who barelly know what it is.
Denmark actually has something similar to this (Sikker mail) but it's mainly aimed at businesses. Based on what I've seen, this resulted in a market for services that bypass the E2E aspect of it because business users can't figure out how to use it. It is also noteworthy that despite this s/mime being available for everyone, Denmark has a public digital mailbox for all citizens and businesses in order to ensure availability.
S/mime is great. It is also not suited for people who barelly know what it is.