Doh. I feel rather silly.
I was checking my email settings on the server I use, just to make sure there were no IP addresses blocked. As I've got several (chinese) ones that create a lot of bot scraping on the website. Nothing untoward there...
But then I realised I'd not checked my filter whitelist and blacklist while there, and sure enough in the blacklist was *.co
Removing that fixed the problem.
Swish FTTP, 900mb/s down, 900mb/s up