Not sure what you mean, but it uses email servers, but if you and your friends are online you use Iroh for realtime communications... So I am not sure, probably just maps email usernames to key pairs.

Btw, Delta Chat just added support for multiple email servers a couple months ago :) I have one main server and a backup as well. The profile data is fully private. It’s shared when you add someone. Btw they are responsive on the forum and on mastodon. Check the latest blog too.