it seems that I found a solution:
i wrote a plugin whick hooks the the 2 functions ‘wpmu_welcome_notification’ and ‘wpmu_welcome_user_notification’
I simply killed the wp_mail calls in them and replaced them with a none-sense function.
Here is my code:
`
function my_nonesense() {
return true;
}
remove_filter(‘wpmu_welcome_notification’, ‘wp_mail’);
add_filter(‘wpmu_welcome_notification’, ‘my_nonesense’);
remove_filter(‘wpmu_welcome_user_notification’, ‘wp_mail’);
add_filter(‘wpmu_welcome_user_notification’, ‘my_nonesense’);