In various tests, many people who connect to port 25 of any of the inbound mail servers for the t-online.de domain are immediately greeted with this error message: 554 IP=[YOUR IP ADDRESS] - A problem occurred. (Ask your postmaster for help or to contact firstname.lastname@example.org to clarify.) (BL)
As the error messages contains (BL) which suggests spam blocking, but it is being reported by multiple, unrelated entities, who are not sending spam or perhaps are not sending much mail at all. I and others were wondering, could be a spam filter be incorrectly configured?
I reached out to T-Online to request clarification. They explained: "In principle, our system does not accept e-mail from hosts with bad or unknown reputation. Hosts with unknown reputation may include such hosts that have not send e-mail for a very long time (longer than at least a year)."
In other words, they're saying that they block unknown IP addresses automatically until you reach out to them to request that email messages from your IP address be allowed through. That seems a bit harsh to me, and against the principle of innocent until proven guilty, but such is life.
If you've been affected by this issue, feel free to let me know in the comments below.