iOS Mail spontaneously using excessive background power?

I noticed this morning that my iPhone 12 Pro Max (running iOS 16.3.1—yes, I know I’m way behind on OS updates, I have my reasons) was draining battery a lot faster than usual. I was down to less than 60% power shortly before noon, after having been charged to 100% at 7:30 am; normally, I’m around 80% at that time. I hadn’t been doing anything with it that I don’t normally do in the morning. I do not normally need to charge it during the day (I charge it overnight), and the battery health is good (98% capacity, optimized charging turned on).

I didn’t really have time to investigate it when I noticed it; I simply plugged it in to charge for a while. After that top-up charge (to about 80%), it seemed to be using normal amounts of power again for the rest of the day.

Later, in the evening, I had a moment to look into what was happening. The Battery usage info showed that in the past 24 hours, 51% of battery usage was by Mail, with 1 hour 6 minutes of activity, all background. The graph shows a pretty sudden, steep decline between 9:00 and 10:00 am, leveling off to more normal drain after that. In the past ten days, Mail’s power usage was 7%, with 2 hours 12 minutes, all background.

This baffles me. I use Mail on my phone very infrequently, only when I need to check something in my mail when I’m out and about. I checked open apps to see if I’d inadvertently left it doing something, and found it several apps back in line, displaying a PDF I had opened from an email sometime last week. I haven’t actually used Mail on my phone since the day I opened that PDF. I have sent a few emails from my phone through other apps via Sharing since then, but not in the past couple of days, nor have I made any changes to any of my accounts. I do not allow iOS Mail to check email in the background.

The only thing that seems to correlate with the time that shows the excessive drain was that I had had difficulty sending an iMessage to my spouse shortly before 9:00 am this morning, in the middle of a conversation, and I had to turn iMessage off and on again to get it to work. That lasted less than two minutes.

What could Mail have been doing, in the background, unprompted, that would cause it to use so much power in so little time? Does activating iMessage create background activity in Mail, and is it reasonable for that to last an hour?

The only thing I can think of is that some network issue (maybe the same othat made Messages glitch) was causing it to repeatedly retry an operation (to fetch or send messages). We know that wireless connectivity is a power-hog.

Otherwise, I’m at a loss here. Sometimes a bug can send an app into a power-hog loop for no good reason. Restarting the app is often the only way to stop it.

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You can try changing settings / mail / accounts / fetch new data to manual, so the mail app isn’t getting updated in the background. Also turn off push notifications for mail if you don’t need them.

But iOS really doesn’t allow for too much analysis of why an app is using power - just that it has. It’s hard to diagnose an issue like that, particularly if it happens only once.

Just a gut sense, but this feels like an indexing operation. Perhaps your toggling of iMessage invalidated Mail’s cache or Spotlight index for some unknown reason, so it had to rebuild.

That makes sense. I don’t believe that the iMessage issue happening right before the drain is a coincidence, and reindexing is a plausible background operation. As I said originally, I don’t allow Mail to check for emails in the background, only manually. I also do not allow push notifications from Mail (which would be pointless if it’s not checking in the background anyway). But this sounds reasonable.