@Pintosal... thanks I'll look into that.
I suspect it may be overkill however, as I'm happy to stay with freeparking for domain registration, and I actually get my top-notch webhosting on by special arrangement from elsewhere so I don't want to disturb that either. Email final delivery is via ISP (demon) which I'm also happy with, it's just that I want mail that's addressed to my personal .com & .co.uk addresses to be forwarded to my .demon.co.uk address for final POP3 retrieval.
The freeparking control panel allows me to forward mail to demon exactly as I want, but it's flawed as it restricts attachment sizes and blocks certain attachment names, supposedly in the interests of anti-spam/anti-virus, whereas I want totally transparent forwarding . But Freeparking also allows me to nominate any IP address I like as the mail server for each of my domains, in which case they (freeparking) never touch my emails, so what I'd like is to find somebody who offers me an address for a simple mail server that just forwards mail to my ISP addresses - no web hosting - no domain reg.
I'm not actually sure such a service exists. Maybe this is a business opportunity for me to start one, Dragons' Den here I come 8)
@oldfogy... as described above, it's the 'man-in-the-middle', who receives email for my domain name and forwards it to my ISP, that is causing a problem. So ISP restrictions, or absence thereof are not really relevant, and nor is there any software I can run on my PC that will change anything.