Once you have created a mailbox for your domain in your VPS 'Mail' section (further details here), the Incoming and Outgoing Mailserver addresses to use in an email client program would be mail.yourdomain.com (where 'yourdomain.com' would obviously need to be replaced by your actual domain) or, alternatively you can enter the I.P. address of your VPS in the Incoming and Outgoing server fields.

Also, please make sure you check 'My server requires authentication' in the Email Settings of whatever Email Program you use (the following example is Outlook Express)



Click the 'Settings' button and leave as the default Login Information 'Use the same as my incoming  mail server' as shown below.