Select in the upper navigation bar of Thunderbird the point "Extras" and click on "Account Settings".

A new window will appear in which you click on "Account Actions" in the lower left corner.

An extended menu will open up:

Click here on "Add e-mail account".

You will now see the following screen:

Fill in the fields as shown.

Your name: Your "natural" name or your company name.
Your e-mail address: Your e-mail address
Password: Your personal password for the respective e-mail account.

And click on "Next".

The following screen will appear:

Replace all fields with the information about your e-mail box.
You can also find this in your customer login under "E-mails -> Server information".

As user name, take the e-mail login of the e-mail account you created when creating the e-mail account in your customer area on checkdomain.net/en was assigned.

And then click on "Test configuration again".

The program then checks whether the settings are correct and logs on to the registered server as a test. 

If you get an error message, check the entered values again.

Now you will see the following screen:

Click on "Create account".

The e-mail account is now set up, but the settings for sending must be changed.
Please click on the "Extras" tab and select "Account settings...".

Scroll down to the bottom in the window that appears on the left side and click on Outgoing mail server (SMTP).

On the right side you will find your domain name or the mailbox you have just created in the small field. Select it so that it is highlighted in blue and click on "Edit" on the right side.

In the window that now appears, the last field, i.e. the username, must be changed according to your username which you have already used in the 4th step. Finish the setup with a click on "OK".

Your new e-mail address in Thunderbird will be created and can be used immediately to send and receive emails.