Only registered users with the right group permissions (see Group settings -> [Click on group name] -> System admin permissions -> Users & Groups) can view user profiles in the backoffice.

The function to email users through a form doesn't reveal the email address and can be turned off in the user profile.
To modify the formatting of the emails you've to edit htsrv/message_send.php.

not true,
if a registered users comments on an other users post, then there email is visible in the backoffice.

You're right, Topanga, I didn't think of the backoffice comment list...

That's a big brainer for me at the moment.
At the moment I fixed it with 'display: no'... offcourse that is no-brainer to find, but at least the emails do not show up 'in the face'

Try commenting out line 62 in inc/comments/views/_comment_list.inc.php:

//$Comment->author_email( '', ' &middot; Email: <span class="bEmail">', '</span>' );

You could also try:

if ($current_User->group_ID == 1) 
{
    $Comment->author_email( '', ' &middot; Email: <span class="bEmail">', '</span>' );
}

which will show the email address to administrators only.