The messages for offline users are stored on their servers and delivered when they connect. Interesting that you list this as a problem because this worked for ages (given server support for offline messages).

Conversations can display a "tick" on the sender's side so you know that a message has been delivered to the user (not just stored on a server).

I guess it works well as I've never heard about a lost message.

You can check the server with compliance tester: https://compliance.conversations.im