I believe MailMate does this by default? I've been using MailMate for a little over a year now and I've fallen completely in love with it.
Last time I checked, although I could prevent image loading in Gmail for desktop web browser, I could not do so in the Gmail iOS app.
They used to be able to tell where you were reading it from geoip, but google killed that by proxying all images through their servers as of a few years ago.
- "A while ago" is about a year ago. When choosing a new email client, run your own test and don't take my stale test results. emailprivacytester.com is fantastic.
- It is not an apples-to-apples comparison. I used Apple's native client as a pure IMAP client fetching directly from my email server, while I think many other apps want to pre-process your emails on their own server sides so they can providing timely email notifications without eating your smartphone's battery for background activity.
Turning in html should be an option done only when really needed.
Who is Steve? Nobody knows, but he's in the “knowing who's reading emails and when” business. It's a shady business. Don't let your computer phone Steve.
A while ago, I used https://www.emailprivacytester.com/ to test several famous iOS email clients, and most of them more or less leaked _something_, even without loading remote content. In the end, I found Fastmail and Apple's built-in iOS mail client to be the top-notch in terms of privacy (Fastmail leaked nothing but only their server side DNS server via DNS prefetch[1][2], which has nothing to do with client. Apple is slightly worse, but still far better than any other email clients like Outlook, Spark, Edison...)
1. https://www.emailprivacytester.com/testDescription?test=dnsL...
2. https://www.emailprivacytester.com/testDescription?test=dnsA...