I get this all the time when I send an email to many recipients from gmail.
I use gmail as my email client but use the send as feature to be able to send as my own private domain name.
This works fine for emails to 1, maybe 2 people at a time. On the rare occasions that I need to send to more than about 4 people I get half the emails bouncing. Very annoying. I've seen similar forum posts Virgin media just need to sort it out.
Hopefully someone who can sort it out will see this post.
Google tried to deliver your message, but it was rejected by the relay smtp.virginmedia.com [188.8.131.52].
The error that the other server returned was: 525 Authentication Denied (VM311)
Apologies for this though I think we can get this resolved. "VM311 - authentication denied 525" indicates too many authentication attempts via IP. This is likely due to Gmail's large sending IP range which I could query with our email engineers in the hopes of getting those ranges updated our side.
I've sent you a PM (purple envelope icon, top right of page) requesting some additional information. Please reply when you can so that I can inspect the SMTP log data from here and get this raised up for you.