sure that is how pop3 does it

but metadata is still replicated to the server, so this does not solve the metadata replication issue

even if you dont explicitly store metadata or encrypt it in some way, the server still necessarily knows stuff like timestamps of when the messages are sent for example.

sure, you can delete it later, but you also have to trust the server to actually do that, and there is no way to guarantee this in any protocol

what alternative do you propose for saving messages when the recipient is offline?

tbf there is not really a good solution to the ‘ephemeral aspect’ problem

the only way to truly not sync metadata to or btwn servers at all is to use a p2p model, in which you cant send anything if one of the parties is offline

simplex might be a bit better in this regard, but still relies on servers for syncing. at least it doesnt extensively replicate metadata like matrix does though

so it depends on your threat model whether this is a compromise or not