The E-Mail Privacy Test make you aware of privacy leaks and security bugs of your email client.
For that, Open the test page and send a mail to your mail account. Read the received message in a new browser tab and go back to the E-Mail Privacy Test page.
https://emailprivacytester.com/
Nice share, as usual.
I just tried this with a secured setup of Pale Moon 24.2.2. Fastmail seems very secure, as it doesn't trigger anything. Outlook's mobile interface fails the first two DNS prefetch tests, but only leaks Microsoft IPs, not my own (the site says this can happen).
Tried with outlook and only dns anchor gets triggered and leaks Microsoft's IP.