Exchange 2003 SP2 for Small Business Server

http://www.vladville.com/articles/exchangesp2sbs2003.asp: "Exchange 2003 SP2 on SBS"

"Yes it is supported on SBS 2003 and Yes, it actually works on SBS 2003"

See also:
http://www.vladville.com/2005/10/more-exchange-2003-sp2-documentation.html
"More Exchange 2003 SP2 Documentation for SBS"