After my original post here I dropped the server XOOPS version back to 2.0.7 and everything was OK. So, at least I knew it was (is?) a 2.0.9 issue.
I then put 2.0.9 back and tried a new registration on my home system. I experienced the described problem, ie I never received the notification. However, the registration details were committed into the XOOPS db.
OK, after some messing about and much gnashing of teeth, I established that the problem is related to users' firewalls.
In my case I use Kerio. Other (complaining) users are using Zonealarm and the WinXP_SP2 FW.
Notifications are NEVER sent if the firewall is enabled. (In my case my firewall has been configured to let thru the domains in question)
However, notifications are sent if the firewall is disabled. But that's not really desirable, is it?
The other thing that I don't understand is how come the problem has only appeared in version 2.0.9?
And finally, and most importantly, how come it's only me and my prospective users? Or are we the only ones using firewalls

Thanks