uservalidationbyemail stops sending emails - help!!!

I've created a new Elgg installation (1.7.8) - I enabled a number of plugins, everything was fine. But the uservalidationbbyemail stopped sending the validation emails.

I tested another installation on the same hosting (godaddy) - everything fine.

I deleted & reinstalled elgg and the probelem was fixed. I then added plugins one at a time and tested the registration after each plugin was enabled. All required plugins installed & all working fine.

I started to customse apparently trivial things like the custom index widgets, completed my profile. I tested the registration and it had stopped working again. I backtracked, and undid everything I did since it was working. no luck.

I then disabled all but the most necessary plugins. no luck.

I tested othe notification emails - all worked all the time.

I deleted the installation & DB to go back to a known working state - now it doesnt work even from a clean install.

The other install on the same hosting still works.

I can't see how to verify the email has been generated & sent. I have no 'sending' folder on the server, or bpunce backs that I can detect. They are not in any spam filters. email address is valid - I get other notifications.

Any ideas? I'm at my wits end!