It is not clear from the Mail tab, that when you create a mailbox, that the full email address is required as username. On my old server, when you created alias@domain.com, then "alias" is the username. It would be useful if there was a line of explanation, that if you create the email address "alias@domain.com" with a mailbox, then the username is "alias@domain.com" (not alias).
Have just discovered that there is a configuration option to change this in Admin | Tools & Settings>Server-wide mail settings.
When I was googling I found this information in the KB of parallels. I think this will help you out so I want to share this with you :- http://kb.parallels.com/1178 __________________ Macbook Pro 17 2.8Ghz; Host OS: Snow Leopard 10.6.2, 32-bit;Guest OS: Windows 7 Ultimate x64... Proud