If you have booked a hosting package with INWX, you can set up an SPF record with the following steps:
1. Log in to your INWX customer area.
2. Open the menu item Nameserver.
3. Click on the icon with the eye next to the domain in question.
4. Click on Add DNS record.
5. Leave the Name field empty. Select TXT as type and enter the following value: v=spf1 redirect=_spf.web-server.biz
5. Then click on Save.