I got an email from freenet.de. 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 <email@example.com>: 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 (freenet.de) without any problems or error messages.
I also created an issue, but unfortunately I don’t know where I should create it. So I chose “mailstack-automx”.