I'm shouting into the wind here a fair amount, but I just discovered DeltaChat and ArcaneChat. ArcaneChat is available only on Android, but DeltaChat is available on iOS and uses chatmail relays. My family has found it to be super useful and effective. I don't think it's viable to convert everybody in my social circle, but I've been very happy with the apps and attachments, notifications, and general performance.
I'm considering running my own chatmail relay, although I haven't tried that yet.
I use DeltaChat just by myself to share things between devices. Not really the intended purpose but it's good enough for me!
DeltaChat was much more useful when it worked with any email server and didn't need its own special kind of server. We have plenty of messengers that use their own servers, like matrix, XMPP, and simplex.