Hello, thanks for the replies!
John Levine via mailop wrote on 05.07.21 at 18:59:
I believe the recipient's mail system is misconfigured. If Cyren tags an IP as yellow, they should soft fail with 450 so it can retry later.
That could be indeed helpful. I was reaching out to their postmaster already (recipient is a larger German city) to see if they can do something - a greylisting would be "fine", but the hard reject is an issue.
Alessandro Vesely via mailop wrote on 06.07.21 at 18:38:
I tried https://www.cyren.com/security-center/cyren-ip-reputation-check-gate and it says "No Risk" for 188.34.176.133 and "Please enter a valid IP" for 2a01:4f8:c010:587c::1. Is it the latter the one that gives you problems?
The IPv4 is one of the several IPs affected, indeed. I unblocked it again last week, but it likely will switch back after a while. Not sure if CYREN supports IPv6 - but the recipient's MX doesn't in any case, so it's not a workaround to deliver the mails either.
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