NOTE: Disregard the info in this post for now. It has come to my attention that this might not always work and therefore requires some more research. Sorry!
With the introduction of Autodiscover in Exchange 2007 this might not be very relevant, but some users still use PRF files or the Office Customization Wizard to pre-stage settings for uses’ Outlook Profiles. Whenever you use one of these methods, or if you create your Outlook profile manually, you will be prompted with a list of users in the Global Address Lists that match your username. E.g. if you username is paulo and another user’s username is paulos, Outlook cannot uniquely identify you and prompts you to select which user is you. This can be a problem for users who are likely confused by the list and also a source of errors since the possibility exists that the user will select the wrong entry. This last case will result in an access denied error since the user does not have access to the selected mailbox.
To work around this you can add the default SMTP domain to the %username% variable specified either in the PRF file or the Office Customization Wizard. There cannot be two users with the same SMTP address in you organization so this will uniquely identify the user. Note that this approach requires each user to have an SMTP address in the form <username>@<SMTP domain>.