Use InPrivate browsing to access it then it will not interfere with your normal google account (If you have one)
You'll find that GMX mail servers may appear on blacklists from time to time (due to service abuse) so emails you are send are more likely to turn up in people's junk email folders vs slightly more reputable services. In addition the GMX interface isn't very good.
Ideally if you can afford it - worth considering registering a domain and using either a web host webmail or an Outlook.com account which allows integration with a domain name. It will cost a few quid but help a lot when it comes to transfer time and also reputability.
Andrews & Arnold Home ::1 on Draytek 2862ac - Why settle for inferior?