t-online.de blacklist "host refused to talk to me" (BL)

Solution 1:

I have finally found out that all the pages I visited during research were talking about the error message without the (BL) suffix. Without "(BL)", the server wants to tell you, that you are blocked automatically and will be unblocked automatically after 24 hours. You should check forward settings or spamming users, as discussed in the web forums.

In our case, the error message had the rare suffix "(BL)", which meant that our server was on the internal permanent blacklist which is a very rare condition and not well documented. Now it makes sense - BL stands for blacklisted, of course. Writing "Permanent blacklisted" would have helped.

After contacting the email address given in the error message, it turned out, that not our server itself was permanent-blocked, but the datacenter, because there were too many spamming users. Due to our request, only our server IP (xx.xx.xx.xx/32) was whitelisted.