I think there's a lot of confusion surrounding this app, so I'm going to make some assumptions and please correct me if I'm wrong.
- This isn't an email app at all, but it's a messaging app that uses the email paradigm to display how messages are received.
- "Deferred messages" simply wait in the app's Outbox folder until your contact comes online and then they will be delivered.
(It's interesting how Delta Chat is technically an email client that pretends to be a messaging app, but this is more of a messaging app that pretends to be email.)