Hi,
bl.spamcop.net is notorious for 'false positives' IMHO it isn't very clever to use bl.spamcop.net without whitelisting [1] 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)
Martin