E-mails

I need help,
I've got one of those websites where you can signup to a free E-mail but as you know, some e-mail address's may be taken, I asked my hoster and they passed me on to devshed. I know it needs to be in PHP.
What I want to happen is say webmster has been taken. when they type in webmster in the box and any old password a windows pops up saying "This username has been taken, please choose another name!"

Thanks sephers.
If it would help can you send the php, and if files to:
webmaster@123-solutions.co.uk