Banner at user registration

Yes, you can either do this by implementing hook_mail_alter() in a custom module or by modifying the mimemail-message--[mailkey].tpl.php file (set the proper mailkey for the template file!).

what is the actual mailkey how can i find it?
I'm unable to find any documentation and see a lot of people questing the same.

When i look at the mail send during the user registration process,
I get the following mail. (due to a Mac Mail conversion thingy its sort distroted) but to illustrate my question.

If I have to beleave the forum talk “mail-user-register-no-approval-required” is the mailkey.

Or am i Wrong?

!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN" "http://www=
.w3.org/TR/REC-html40/loose.dtd">
<html>
<head><meta http-equiv=3D"Content-Type" content=3D"text/html; charset=3Du=
tf-8"></head>
<body id=3D"mimemail-body" class=3D"mail-user-register-no-approval-requir=
ed" style=3D"background-color:red;">

mail-user-register-no-approval-required
    <div id=3D"center">
      <div id=3D"main">
        <p>test10,</p>
<p>Bedankt voor uw registratie bij iTstages.com. U kunt nu inloggen bij<b=
r><a href=3D"http://localhost/user" title=3D"http://localhost/user">http:=
//localhost/user</a> met behulp van uw gebruikersnaam en wachtwoord:</p>
<p>gebruikersnaam: xxxx<br>
wachtwoord: xxxxx</p>
<p>U kunt ook inloggen door op de onderstaande link te klikken of deze in=
 uw<br>
browser te kopi=C3=ABren:</p>
<p><a href=3D"http://localhost/user/reset/22/1325017533/xxxxxx=
ad9c3d8770bc8a6" title=3D"http://localhost/user/reset/22/1325017533/1ff04=
xxxxx">http://localhost/user/reset/22/1325017533/1f=
xxxxx</a></p>
<p>Dit is een eenmalige login en kan dus maar =C3=A9=C3=A9nmaal worden ge=
bruikt.</p>
<p>Na het inloggen wordt u doorgestuurd naar <a href=3D"http://localhost/=
user/22/edit" title=3D"http://localhost/user/22/edit">http://localhost/us=
er/22/edit</a> waar<br>
u uw wachtwoord kunt wijzigen.</p>
<p>-- het iTstages.com team</p>
      </div>
    </div>
  </body>
</html>

Yes, it is. I don't think it is that hard to find, you can see it in the message or you can check it in the template preprocess function.