I have the problem that mails from opalstack servers are refused from a big german mail provider. See below:
This is the mail system at host smtp1.de.opalstack.com.
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
firstname.lastname@example.org: host mx00.emig.gmx.net[18.104.22.168] refused to talk to
me: 554-gmx.net (mxgmx015) Nemesis ESMTP Service not available 554-No SMTP
service 554-IP address is black listed. 554 For explanation visit
Following the given link, it tells me the reason "5xy IP address is black listed"
Any ideas or solutions?