Just had a situation where I had to send 29 faxes from a Windows XP Pro SP2 box to a Windows 2003 SBS server.

So I did the following:
Scanned the item I need to fax to 300dpi tiff format.
Import the names / address / fax details from an Excel spreadsheet into Outlook and place in a seperate contacts folder.
Open the tiff in "Microsoft Office Document Imaging" application and choose file ==> print ==> \\SBSServer\fax
In the Recipient Information window of the printing wizard select all the names from my Outlook contacts
Remove the "Select Coverpage..." option leaving the default send schedule options of now.
Finish the wizard.

The result:
29 "pending" faxes that never went.

The problem:
It appears when you send a heap of faxes to the SBS server it gets busy putting the faxes in the send queue and the send "now" time window never gets triggered.

How to fix the problem:
When you are going through the wizard choose a time to send several minutes in the future. The fax service will then have time to recieve the many faxes and trigger and begin sending the multiple faxes as expected.