thank you knebb! I found the relevant FAQ from my domainhoster too. It is now possible to change the PTR record. So I will do that.
But before that, I need to be clear on what fdqn I will tell the domainhoster. (Is it even relevant as long as it reflects mydomain.country_xy for my IP4 address?)
There is still the topic of the mismatch between the postconf myhostname in UCS and the actual MX record. Also, letting myhostname unchanged with another MX record/smtp_helo_name exposes host.int.mydomain.country_xy anyway? At least if I look into email headers. So what is the point here?
So I either leave host.int.mydomain.country_xy everywhere or change it to mail.mydomain.country_xy (same IP of course) or …?
Regards,
Bernd
Edit: I have posed the question again here (Postfix myhostname 4.2) as the GMX problem and solution is solved here - thanks to @knebb.