I'm using F-droid K-9 and am happy with it. What would be the point of switching? If the new thing is flat-out better then it might be worth considering, but I haven't seen any explanation of this.
IMAP IDLE push support for any server that supports it, good configurability in settings for a wide range of advanced power user features, and good private-by-default settings (avoiding and blocking tracking pixels/images in emails, and stripping known tracking parameters from links you click, etc.)
IDLE is especially difficult on recent Android versions without a hosted service piping through FCM. You may be able to get away with a foreground service that displays a permanent notification, but I believe Android 11 or 12 limits even those services so you cannot have a persistent process keeping a socket open.
Power usage would be an interesting problem as well, because I'm not sure if keeping a socket open would pin the radio active.
I haven't had any issues on Android 11 with FairEmail using a foreground service. I believe that the changes coming in 12 are able to be worked around though, and at minimum a version distributed outside of the Play Store won't be affected. There's some get-outs to Google's changes at least as they stand right now, but who knows if that will remain the case in subsequent versions.
I haven't seen any suggestion the radio is being pinned active - it seems to still sleep fine, and I believe an incoming messages triggers a downlink page to the phone, which will then wake the radio to deliver the packets.
Doesn't k9 already supports IDLE? I kinda remember researching about this years ago, unfortunately, IDLE on it's own is no equivalent to native push notifications that use google services. Either way, it was just a hobby research and I might be wrong.
Last time I looked into this, K9 didn't work in background continuously (since Android requires displaying a notification for that, and K9 never implemented it). And nothing seems to be happening in this regard.
However, I wasn't able to make FairMail to deliver without a considerable delay either—despite it displaying a notification and being excluded from ‘battery optimization’. But apparently it's just me.
Out of interest, do you use a phone with a custom OEM firmware that has battery-saving features. I've seen issue with FairEmail (and other well-behaved foreground-service using apps) on some OEMs' firmwares. Heck, sometimes even big-name OEMs like Samsung cause issues.
On "pure" Android (thinking Pixel, and the relatively pure Motorola devices, etc.) there's no delays at all - it's really impressive. I'll get the notification on my phone consistently before on my PC.
It's a Pixel. Starting with 9 or 10, Android limits apps' background activity unless they show a notification (and probably has limits for those too when the screen is off). Plenty of apps had to deal with this, I have four of these notifications.
These limits don't apply to notifications via Google's services (GCM or whatever they're currently called). So if you're using the Gmail app or another app for a specific mail service, sure you will receive push notifications quickly.
Interesting - I've not been able to reproduce this on a Pixel 3a or 4a 5G. FairEmail, with foreground notification, doesn't have any issues delivering notifications.
As you say, Google is making it a lot harder, but FE with a foreground notification seems to do this fine for me.
EDIT: See comment below this - it appears I might be talking out my bottom.
It's easy to change an app's background ability in Android. By default apps are forced into the background, but if you change that your k9 email will notify you all day long (it does support IDLE)
There's an issue on their tracker that discusses implementing the notification, and docs that say that without it K9 can only check mail every 15 minutes or so, even if excluded from battery optimization. So I don't know what support for IDLE and ‘background ability’ you're talking about. Before Android 9 or so, sure—it could just run in the background and connect to the servers whenever it wanted.
I love fairmail. Can't remember what the problem was with k9. But fair mail can't do everything. And it's slick and modern. Privacy first. Removes tracking images. And also identifiers in images you send out. It's really really good
P.s also f-droid here (ps you can donate with crypto if you want)
I don't understand the comparison? Outlook and Thunderbird were pretty similar iirc. K9 does give me notifications when new email arrives. It's possible that it's every 15 minutes rather than instant: it hasn't been an issue for me so far. I know that Linphone is able to receive phone calls which is presumably by listening on a socket or getting activated by some inetd-type thing in Android. Maybe K9 could do similar.
