Email can go better than APIs - it has publicly available protocols and there are many implementations of them (including many good FOSS implementations). They are called IMAP and SMTP! Any email client can integrate with any email server, and has been able to since the beginning of email
Sounds wonderful until you have actually tried to integrate IMAP "supported" email services, such as Gmail. What makes it hard is that different providers have different implementations which very often do not follow the protocol spec. Microsoft, Google, Yahoo all have their own higher-level API for email access, and every modern email tool ends up making something like Nilas, with a server taking care of pushes and managing logic for each provider. The industry needs a new standard, and that is why ideas such as Nilas seem exciting.
