The kind that runs locally, or that is run by my trusted (by me, by my employer, ...) email provider. Email is just too valuable to be exposing to extra third parties.
Phones have made this more inconvenient because it's hard to make 'the app that runs locally' run whenever 'the app that views my email is running'. That means running locally is a big constraint on the UX, for example it probably can't reliably sort emails before my phone picks them up. It's simply the case however that UXes that don't work with local apps aren't viable products to sell to me.
Open source not running locally is closed source, for all intents and purposes.
A GNU/Linux box supposedly loaded with nothing but software libre is closed source, if it's remote to you and you don't own it. If you send your data to it to be processed, who knows what happens to it.
I hear what you’re saying, and I agree that running something locally is appreciably different than using something hosted elsewhere.
But I think it’s critical to be precise about this. Open source is still open source, and this designation is still meaningful even if an open source product provides a hosted version. The downsides of letting someone else host something are independent of the the open/closed source status.
Yeah, and I briefly considered that, but in truth I can't be bothered to run software stack of this complexity for my personal use for this. It's designed as a mutli-user web app, and that shows in the number of different services used in the backend. I like the idea of an app that sorts my email better, but not that much.
I would say I don't consider this a criticism of the product - they're clearly selling a hosted product that they were kind enough to open source, not making a product for end users to run that they happen to sell a hosted version of. It being inconvenient to self host doesn't reflect poorly on them.
Phones have made this more inconvenient because it's hard to make 'the app that runs locally' run whenever 'the app that views my email is running'. That means running locally is a big constraint on the UX, for example it probably can't reliably sort emails before my phone picks them up. It's simply the case however that UXes that don't work with local apps aren't viable products to sell to me.