[MM3-users] Flooded non-members with "prvs=<number>=<mail>"

Dears,

I checked the non-members list of all my mailman3 lists and found a lot entries like "prvs=<number>=<mail>", the <mail> is always an address from @freenet.de

A quick research mentions something related to bouncing and that such mails should be blocked/filtered. But no further details.

Can you give a hint where I need to change what to block such mails and avoid that such entries floods the non-members list?

Thank you and kind regards Torge