As a thought for people using multiple addon domains. If you create a subdomain to an addon domain, it still appears as a folder under public html (so it seems). So if you have as addons
domain1.com
domain2.com
you will have these as folders within public_html. If you were then to create a subdomain on domain1.com called webmail you would have the folder 'webmail' created within public_html.
However, if you then create a subdomain on domain2.com called webmail it would simply overwrite the first webmail folder, as you can only have one thing with a single name.
So in this circumstance, create a folder named webmail inside your addon domain folder eg
public_html/domain1/webmail
public_html/domain2/webmail
Now this obviously doesn't immediately help you. But now you can have them separated properly. So now you need to create a simple index.htm file with the following contents:
<html>
<head>
<title></title>
<META HTTP-EQUIV="refresh" CONTENT="0;URL=http://www.domain.com:2095/horde/index.php">
</head>
<body>
</body>
</html>
Of course you can make this as fancy as you like. But remember it will only show for a couple of seconds - up to about 5, and you may like to add a text link, for those using browsers that don't support auto redirect.
Finally if you don't want to use horde, you can opt for the others