To be fair there's not much to do with it. Market share wise there's Thunderbird followed by a large chasm and then maybe Protonmail and other clients with plug

To be fair there's not much to do with it. Market share wise there's Thunderbird followed by a large chasm and then maybe Protonmail and other clients with plugins. S/MIME is significantly more widespread, but has a cost of entry, it's fairly undocumented and quite a few clients have buggy implementations.