kylotan we're aware that smtp2.de.opalstack.com's IP address is part of a larger IP range that is presently listed in the UCEPROTECTL3 list.
As you noted, this is not a problem specific to Opalstack, ie the IP for smtp2.de.opalstack.com is not listed - rather, the larger network to which it belongs is.
There has been an increase in this sort of listing since January 2021 when UCEPROTECT changed the way they operate. In the past, they operated mostly be collecting IPs that were sending mail to their spamtrap addresses. In January, they began listing IPs that they associate with other types of abuse such as DDOS attacks. As a result, if a single IP is caught by them, that IP goes on UCEPROTECTL1 and the network range goes on UCEPROTECTL3.
Since it's not our IP address that's listed there's little that we can directly do ourselves to correct this. We've already let our providers know about this problem and they've stated that they're doing all they can to clean up their network to get their IP ranges off of UCEPROTECT.
The only other option is to move the affected parts our mail infrastructure to a new provider, but it seems that no provider is immune to these problems brought on by the changes made by UCEPROTECT so we're not planning any moves at this time.
Note that you should never see a message bounce as a result of the UCEPROTECTL3 listing, at least not from a reasonable mail provider. If you do, then please email Opalstack support with the details.