I don't currently use QuickBooks, but this seems a common issue. Seems you have to pay for their "Billing Solutions" interface and then you can track mail.
http://quickbooksgroup.com/webx/foru...ase_orders/312
Also it seems that this has been added to 2008 versions of QuickBooks or allows the use of Outlook.
http://www.sleeter.com/articles/1192558999