The above error means that the mail from gmx.de was rejected due to spf setting. The sender domain: gmx.de have SPF setting that do not allow to send the email from the IP 18.104.22.168. The spf is to prevent any mail forgery, spam issue, email spoofing and etc. The basic SPF setting is v=spf1 a mx -all. To confirm if this string exists, log in to your cpanel>Advance DNS Zone then check for a string which looks like:
gmx.de. 14400 IN TXT v=spf1 a mx -all Edit Delete
[v=spf1]: This identifies the TXT record as an SPF string.
[a]: refer to the domain IP . The domain is allowed to send mail through this A record
[mx]: refer to the domain MX record. The domain is allowed to send mail through this MX record
[-all]: No other servers are allowed to send mail from this domain
So you can click on delete to remove the string from your account first before you be able to send and receive mails normally. Once its deleted the issue will be solved.
I hope this helps.