is notorious for 'false
IMHO it isn't very clever to use bl.spamcop.net
whitelisting  some known 'clean' providers...
It is possible to get whitelisted at spamcop.net
GMX was also listed at spamcop.net
.. and i wouldn't refuse
any mail from GMX, even if they are on a blacklist..
Any provider can call them (as we did) and tell them they
should whitelist the providers MX-Ranges. Since we did that,
we have no longer problems (we are not listed there over 8 month)