I just sussed it - this spammer has a valid email address buried within the usual 20 or so 'try ons', so the pop server is correctly rejecting the 'not knowns' and passing the valid one.
No catchall configured, interestingly when checking the logs 90+% of the spam doesn't get through just the odd few. The POP box reports these as having been refused - very strange as not consistent.
I know this is a free account and I have to say the server response etc is great.
However, some spam email is being passed from the pop server to my email client addressed to random 'names@mydomain.com' even though when the email logs are checked the record shows 'no such user here' , message...