Make sure your user name and password is all lowercase.
Make sure you have created the email address in your control panel.
Change the user name to the full email address. Example: username@domainname.com
Still not working?
Change your incoming and outgoing mail servers from mail.yourdomain.com to the ip address listed in your welcome email.
Are you able to receive emails […]

To create an email account login to your control panel and click add/remove email account’s. *NOTE: By default you have no email address setup. The main username may look like it’s a setup email, but if you wish to use it you will need to add it as an email account.