The problem that is currently happening is that one of the two DNS servers run by Barracuda is stale (shown in the image below). This is causing email servers that are doing lookups to show some people as being listed even though Barracuda considers them as not listed. This is why their web lookup tool will show them as clean, but the listing is still being published via DNS.
For a refresher course on How Blacklists Work you can check out one of our past blog posts.
We’ll update this post as we get new information