Usage telemetry can be gathered pretty easily from the client itself. Having an intermediate service is usually for stuff like push notifications, search, autocomplete, recommendations and really any other feature that goes beyond the email spec and would be difficult or cpu intensive to implement on all the clients.

Pessimistically for telemetry, but I suspect some of them are Javascript apps in a lightweight shell that have actual technical difficulties making a TCP IMAP connection.

Yeah, I think difficulty making an IMAP connection is one reason. I think the biggest reason has been to decorate more features on top of the backing email service, like adding Snooze support to an arbitrary IMAP account (so the server can un-snooze a message at the scheduled time).

That being said, there are security/privacy implications to that model, and I wouldn't personally use an app that works this way. That's why Mimestream was built as a traditional client.

Doing TCP from a "native" JS app should be pretty easy[1] (caveat: I haven't tried.) I'd think the cost of running a proxy server would be enough motivation to figure this out, for any developer whose primary reason is as simple as technical difficulties with JS and TCP. I think it more likely the other responses such as doing heavy indexing that won't work as nicely on the clients.

[1] https://stackoverflow.com/questions/41674063/is-it-possible-...

On iOS (and some of the more aggressively power-saving OEM Android builds), it's the only way to implement push notifications

On the positive side, you could use them for some privacy features, like predownloading images (works with popular clients like Gmail which prefetch them, whether you read the email or not, to water down analytics)

It turns the app into a service which might require an ongoing fee, which is revenue.

Indexing and search can be sophisticated & available before downloading any messages; you can ensure completion of complex pop and imap operation (eg working with messages "tagged" by gmail); just a few benefits.

Note, this is in no way an endorsement of middle manning email management and facading as a client.