Does this prove it is a Virgin problem or just something being flagged by Virgin?
I think it is worth noting that your screen shot from the MX Header Analyzer does show that:
1) there is an issue with the sender's SPF record (the red cross against "SPF Alignment" in the shaded block towards the top)
2) in the Relay Information section, the "Blacklist" column extreme right shows that the Outgoing servers in steps 1 and 3 are flagged as appearing on a blacklist (blocklist).
Either or both of those factors may well have been sufficient for Virgin Media to have been suspicious about those emails. VM do seem to be very fastidious about issues like that.
On the other hand my thought is that Gmail is probably less concerned about issues like that but probably relies much more on its spam filters to identify suspicious emails based more on the content of the email. I stress though that's just my personal opinion based on experience with my own Gmail account.
As @用心棒 as said, the Forum Team can take this up and get it investigated.
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