This is why I suspect that there was a problem at the Virgin SMTP server end... was that the case?
I’m afraid not, no, the problem is still at your end! You have a device on your home network which has picked up some male ware and is sending out spam. This has caused you to be listed on one of Spamhaus’ block lists and as such VM prevent you from sending email. Often, in an attempt to evade detection, such malware is programmed to periodically go dormant and ‘sleep’. Then no more spam is detected from your network, after a couple of days, Spamhaus delist you, and you can send email again - well, until the software fires up again and the cycle continues.
There are a fair number of posts on here from people reporting the same thing, it ‘just fixed itself’ and often there is a post from one of the VM employees, saying how glad they are that you have resolved this…Expect it’s not been resolved, it’s a temporary respite, until and unless you are able to identify which device on your network is the root cause and deal with that, then, I’m afraid it won’t be resolved.