When Sending emails through Kopano some emails are not send and the user gets a “bounce” message back. This takes a couple of days until the bounce mail is seen.
The “bounce” email will look like this:
This is the mail system at host ucs.mas.intranet. 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 mx01.targetdomain.de[188.8.131.52] refused to talk to me: 554-targetdomain.de (mxeue112) Nemesis ESMTP Service not available 554-No SMTP service 554-Bad DNS PTR resource record. 554 For explanation visit https://www.ionos.com/help/index.php?id=2425&ip=184.108.40.206&c=rdns
For security reasons and spam protection the target domain only accept emails from hosts which have a valid DNS PTR-record pointing to the hostname. Emails will only be accepted when the reverse PTR record matches the forward A record.
Create a valid PTR record dfor your public domain name. Ask your Internet provider to do so. This is not and issue of Univention.