I turn off images

So do I.

The extra protection I expect DuckDuckGo's to provide, is link protection:

Companies embed trackers in images and links within email messages, letting them collect information like when you’ve opened a message, where you were when you opened it, and what device you were using.

So, if you already never click on any links in emails, then DuckDuckGo's utility lies in its ability to generate new addresses with ease whenever you need them. (Something I've been doing for years, but only semi automaticly.)

Trying out their service now...

Trackers get installed in links too of course. From the marketing gurus:

Invisible image pixel allows you to detect the exact time and date that an email has been opened, allowing you to see the bigger picture of what the reader responds to and how you can cater to their needs better.

You can also add redirects and special tags to the links in your email, which makes it easy to collect data about what links are clicked, how many times, and by whom.

Loosely related:
If you use a Mac, in general there's LittleSnitch which can alert you to network calls and requests. It's pretty persistent and in-your-face but I have found really useful to tell you what's going on 'under the hood' of any app, site or service. If you are compelled to use Apple hardware of course.