In other words, the emails aren't being blocked they're being repeatedly delayed with what I presume are 421 errors? Would this be a fair assessment?
As an IT professional you should be aware of the importance of giving clear information that describes the problem properly. While Virgin Media's mail platform DOES block blacklisted servers with a 5xx error - "Your IP is in the RBL", They also indulge in greylisting. Which is a completely separate issue altogether.
If the mail was being blocked with a 5xx error then they would never enter your delayed queue.
I'm going to as @Jen_A to take a look at this thread as I know she's been cataloging and feeding back issues like this to Virgin Media's email team. If you have any example logs, even if the emails aren't in the queue, you should still be able to access those, she will need to see them, don't post them on the main forum as this is primarily a community forum.
And yes I'm aware it may seem as if I'm being pedantic. But when you're dealing with email problems, it is important to realize that the two problems I mentioned have two different solutions.
Only use Helpful answer if your problems been solved.
As per raven's request, if you the reported email errors recur please send me a PM (private message) with a copy/paste of the error message and either a header of a failed delivered mail (if it fails rather than is delayed) or a list of recent SMTP logs detailing the failure/date&time/recipient and sender domain or IP.