I recently learned about a new messaging app called Delta chat. It is based on top of SMTP/IMAP and works with an existing email account so you can chat with anyone who has email already (everyone?) regardless of if they have Delta chat.
Basically, it is an email client that uses a UI similar to most chat apps. I think this is better than something like a Jabber client because nobody actually has Jabber accounts anymore, but email is still ubiquitous.
There are some problems I foresee:
- Many free email providers limit messages to 200-300 per day, which may be a problem for some.
- Depending on the email server messages can sometimes take quite a long time to be received.
This isn't the first version of this idea that I've seen. There are other apps like MailTime but Delta chat is open-source at least and has mobile and desktop clients.