No it is not blacklisted anywhere. The odd part about mail being stuck in the queue is that, for example:
Yesterday, I had about 15 mails dating from a range of about 3 days. I downloaded a few and viewed in a code editor to see where they were from and going. Some of the "receiving" addresses I sent a mail to at that moment, and they arrived just fine, while the others remained in the queue. Same destination, mail size very small. Strange.