Reply to Freenet.de gets a mailer daemon

Hello,

I got an email from freenet.de. The I replied to this email and get a mailer daemon.

Undelivered Mail Returned to Sender

This is the mail system at host mail2.ecloud.global.

I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can
delete your own text from the attached returned message.

                   The mail system

<xxx@freenet.de>: host emig.freenet.de[2001:748:100:40::8:117] said:
    550-Inconsistent/Missing DNS PTR record (RFC 1912 2.1)
    (mail2.ecloud.global) 550 [2a01:4f9:4b:5387:d00b::2]:35058 (in reply to
    RCPT TO command)

I cross checked with an other email provider. I can write emails to the same account without any problems or error messages.