OK – thanks for confirming!
I believe the email in question is actually generated by a pluggable WordPress function – wp_new_user_notification() – and because it is pluggable you can write your own replacement that generates a more tailored email.
Unfortunately, though I could certainly be mistaken, I don’t see any filter hooks (other than perhaps the wp_mail hook) that would make it readily achievable to change things without that step, though perhaps you can find a suitable plugin to save you some work?
In any case, this really boils down to customizing and adapting core WordPress functionality rather than anything directly managed by our own plugins – so I hope the above gets you on track or at least gives you a few ideas you can explore, but it would probably be best to take any further questions to the wordpress.org forums.
With that in mind I will go ahead and close this thread, but I do wish you luck and thanks again for posting 🙂