LJ's IP is listed in the Spamhaus Blocking List because LJ is allowing Russian pharma spammers to abuse their service. Spamhaus is one of the most respected anti spam organizations in the world, and being listed there means they've ignored spammers on their network for quite a while and virtually no large ISP/email host wants their mail until they start acting like responsible Internet citizens.
So that would be the mysterious "third party"? Uh oh.
No wonder I'm getting my email notifications 4 and 5 days late... or not at all.
And yes, I have been reporting and banning a lot of spammers in my comms. Just another reason I'm glad I have my DW account in conjunction with LJ.