A common complaint among some Exchange server users is that their Exchange account keeps getting reset as default. This is a "known issue" if the Exchange server is running on Microsoft Small Business Server 2003.

There are several ways to correct the problem, including removing the PRF (Outlook profile file) from the logon script or creating a new profile using a very unique name. there is also a registry entry you (or your admin) can set to prevent the SBS login script from changing the transport order:

Key: HKLM\Software\Microsoft\SmallBusinessServer\ClientSetup
Value Name: NoTransportOrder
Value Type: REG_DWORD
Value: 1