[Alex3917]

It depends what you mean by email. For example, FWD:Everyone (fwdeveryone.com) allows using the Gmail add-on as the only interface between the user and service. So the user can stay entirely within their inbox, but they aren't interacting with the service by sending or forwarding an email.

[cucho]

Thanks. By email I mean send to an email address, and something happens. FWD:Everyone is more a add-on to a tool (that happens to send/receive emails) than an email-as-interface service.