Downloaded Mcafee on both devices, both say nothing on them ...also, might be a silly question but why would it let me send from either device on 4G/hotspot but it only complains when it’s wifi? Am starting to give up with this...
Emails are being sent from your home IP address that are causing it to be blacklisted on the CSS, this could be sent from any device on your home network. Note Virgin Media has nothing to do with the blacklisting
The CSS listing is causing Virgin Media to reject authentication attempts from your home IP. When you are away from home or using a 4G network - you send from a different IP so Virgin Media allows the authentication attempt.
That the blacklist entry has returned means you need to have a closer look at everything on your network.
Besides using McAfee, I would also consider using Malwarebytes (Free edition) to scan your systems.
As a Very Insightful Person, I'm here to share my knowledge. I don't work for Virgin Media.