Gmx.org (GMX Freemail) doesn't provide open tracking or read receipts via their webmail interface, but they support IMAP, which means you can use another email program like Mailspring to send messages with read receipts and open tracking enabled.
Follow these steps to send email with read receipts from your Gmx.org account:
Mailspring is a free desktop email application and allows you to send email with read receipts and link tracking, and see notifications when recipients open your emails! It works on Mac, Linux, and Windows.
Run Mailspring and choose IMAP / SMTP on the email setup screen. Enter the connection settings for your Gmx.org account and click "Continue". Mailspring is a full email client and allows you to check your Gmx.org (GMX Freemail) email without using the webmail interface. Your email should be displayed in a few minutes.
When composing an email in Mailspring, check these icons in the composer to enable read receipts and link tracking. That's all you need to do!
When your emails are read or links are clicked, you'll see notifications in the top left of the main window. You'll also see icons and tags in your Sent folder, so you can get more information at a glance.
Mailspring even allows you to see who opened your email when you include multiple recipients.
Click your email provider below to learn how to connect to access your account with IMAP: