Al3xios I've used it mostly for testing with friends, but I like it a lot. It has all the same properties you inherent from encrypted email: no forward secrecy, asynchronous/not-real time messaging, inability to support modern chat UX (typing indicators, read receipts), can be delayed seconds to minutes per message due to email infrastructure complexity/network hops (or dropped outright by spam filters. To DeltaChat's credit, they list these drawbacks on their site and design documents and don't try to pretend like they're anything else but an email client with Whatsapp UX.
Signal is better for 99% of your expected use cases for encrypted messaging, and if you absolutely need to send an email it can be encrypted with PGP. But you should still check it out, it's fun. I like that you don't even need to sign up for an email account with their provider and can just use a normal IMAP account.
tumbord It's just Email and PGP bolted together with a UI, you can message to/from it with PGP and vice-versa, so it's no different than just using Thunderbird with PGP.