Well, it has happened again! 2 domains 'bouncing' again. With my hosting company we have checked ALL spam blacklists for my ISP IP, their email sender IP and domain names and we are clear. I have sorted it (temporarily) by changing the SMTP from the domain SMTP to a gmail sender, but I await the next toys coming from the cot.
It appears that Hotmail have some renegade spam system which fires off all on its own for no reason. As my hoster points out, hotmailers may well be missing vast quantities of emails because of this.
Grr!