When an Administrator attempts to create or delete a user within the SBS 2008 Console they may get an error and a  "View Warning Details" link. Reviewing the details show two warnings: Administrator <Admin name> does not have Exchange administrator rights; Mailbox does not exist for recipient of welcome mail. You will also get the following errors in the C:\Program Files\Windows Small Business Server\Logs\Adduser.log file.

Source:→ SBS blog