Gmx.de (GMX Freemail) doesn't allow you to schedule send reminders or follow-up reminders via their webmail interface, but they support IMAP, which means you can use other email programs like Mailspring to schedule reminders and provide a more powerful email experience.
Follow these steps to send email with read receipts from your Gmx.de account:
Mailspring is a free desktop application for checking your mail that allows you to add reminders to emails you send. It also comes packed with other great features like read receipts and thread sharing! 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.de account and click "Continue". Mailspring is a full email client and allows you to check your Gmx.de (GMX Freemail) email without using the webmail interface. Your email should be displayed in a few minutes.
When composing an email in Mailspring, click the Reminders icon and pick a time when you'd like to be reminded about the thread if no-one replies. That's all you need to do! When you send the email, Mailspring will automatically schedule a reminder.
When the time you specified has passed, Mailspring will bring the message to the top of your inbox if none of the recipients have replied. This makes it easy to remember to follow-up, so you'll never drop a conversation again!
Click your email provider below to learn how to connect to access your account with IMAP: