You can easily access your WebMail interface by just visiting: http://www.yourdomainname.com/webmail, where 'yourdomainname.com' is your actual domain name.
How to send e-mail messages via webmail?
You can easily send email messages through the webmail client you have chosen. Just find the New message button in Horde, the Compose button in Squirrelmail or the Create a New Message button in RoundCube and you will be able to start writing your message.
Make sure to SAVE your message if it is too long or you want to write it for a longer period of time. The web mail program may drop the connection and you might lose your email.