Frequent updates to the matching data used by spam filters is likely to be responsible for this and will hopefully clear itself in 6, 12, or 24 hours.
If the problem persist then you will need to wait for a forum team member to liaise with the spam filter provider (Cloudmark) to determine the cause however be aware it can take them a few hours / days to work through to your post.
Good to the read the issue resolved itself. If it happens again post the unique identifier from the error message here to allow Virgin Media (via Cloudmark) to identify and report that cause back to you.
if the board allows, post the (redacted) body of the rejected email to see if folks here can spot the offending words or phrase or link that is triggering the false detect
or do what I have done in the past when some systems don't like some messages - keep removing bits of the message and re-sending it until some stripped down version works ( or until you trigger a too many messages block ). I've done that with pernickety ebay messaging