Mail sent to hosted domain addresses is delivered to the equivalent username.plus.com mailbox and can be accessed by logging in to webmail using the appropriate username-based mailbox and password information.
When sending mail from that mailbox using webmail your can define whatever identities (sender addresses) you wish including domain-based addresses. You choose which identity you want to be the default. Identities in webmail are set in the Identities tab in Personal Settings.
David
Ahh thanks. That's great news. Been having some issues due to RIM's new policy so I may need to use web mail from my phones browser to access and send email.