Use another email provider is gmx.de stops our emails:
Final-Recipient: rfc822;
[email protected]
Action: failed
Status: 5.5.0
Diagnostic-Code: smtp;553-5.7.1 {mx016} The IP address of the server you are using to connect to GMX does not have
553-5.7.1 a DNS PTR record (reverse record). The recipient does not want to receive
553 5.7.1 mail from such servers.