I've been using Linux for years, but only used GUI email applications like thunderbird or KMail. I'm aware there are some TUI/CLI email applications as well.
However, as far as I know, none of their storage formats interoperate with one another. It is a pain that sometimes I find my mostly-used email application is inconvenient for some tasks but I can't switch to a different one easily (needing to reconfigure accounts and downloading emails).
Naturally, a "solution" come to my mind is to have the daemon/service and front-end/user-interface separated -- the email daemon handles the actual tasks like receiving and sending emails, while UI just interacts with user and returns tasks to the daemon.
One well-known example of such architecture is MPD and its clients for music playing. This also helps save memory when the front-end is not running while emails are still being fetched/sent (and notifications, if exists).
My question is: are there such applications for emails on Linux/*nix?
I took a read of the Wikipedia page of comparison of email clients, but found nothing -- they are all stand-alone applications, as far as I can tell.
Surely being FLOSS is the best, but the existence is more important for the sake of this question.
Note about KMail: KMail seems to have a daemon part, but it is tightly coupled with Akonadi and KDE, and there does not seem to be documents about creating other frontends. The existence of a daemon for KMail seems like a coincidence rather than an intentional design (in terms of "being a daemon/service and any compatible front-ends can use it").