As I’ve understood, Delta chat is based on the IMAP protocol and uses the infrastructure of your email provider. Thus, it uses no own server infrastructure, but has the also the downsides of the protocol and some issues with many email providers.
Wikipedia.de - Delta Chat (no English version available yet)
Usually, you notice it, as Mastodon automatically adds an @username@instance for everybody the commment replies to, Lemmy doesn’t.