When building a brand new SBS 2003
When building a brand new SBS 2003 you need to ensure that you install Exchange sp2 (or if you have R2 media, install that) as until you get to the “R2” level of the box, merely running Microsoft update is not enough.
Expecially if you are building or rebuilding with a RTM copy running MU after it is built (Microsoft update for those not into patching or cows) will not get you up to fully patched. The five part SBS 2003 sp1 bundle will not be installed.
And for the record you CAN install the R2 piece AFTER you install Windows 2003 sp2 on the system.