Email experiments: filtering out external images

2025-12-29T14:10:00Z

I had a realization the other day that, on almost every email in my inbox, my mail client has a "show external images" option. Most email I receive references externally loaded images via HTML. There are good reasons for this (not needing to send a copy of an image in every email on a newsletter) as well as nefarious reasons (the remote server can track where/when you load the image). So most mail clients I've used don't load them by default for obvious privacy reasons.

This got me thinking about the reverse, though: what email do I receive that does not include external images? And the answer is, mostly, email sent manually by a real human! I'm fairly certain I've never sent an email to another person in my personal life with an externally loaded image in it. When I have, it's been work email with a standard corporate email signature (that I'm sure was being tracked, natch). Mostly, when sending images to someone, they're sent as attachments to the email.

So I had a realization that, if I wanted to naturally filter email that was sent by hand from email sent from an automated system, this might be a decent proxy for that. Here's the sieve rule I landed on for now:

if body :regex "<img[^>]*src=\"https" {
  fileinto "Inbox.Automated";
}

(Yes, it's typically folly to regex on HTML. This is a simple enough match though that I hope it's fine!)

So far, after a day or so of usage, it's been shockingly effective. There's enough potentially important stuff in my Automated folder that I will need to check that fairly frequently, but the one email that's made it through this filter into my normal inbox is one sent by hand to a mailman list. Honestly, I'll take it! This makes the volume of email in my normal inbox much much more manageable, and I expect most mail that lands there will be things I actually do want to pay attention to.

Probably you'll want to have another rule that prevents you from running this for email from people in your contacts. I haven't done that yet, but that'll be a natural next step.

This is part of my December Adventure 2025 series.

Terracrypt

Email experiments: filtering out external images