Outlook Prompt caused by Excel Macro

In Office 2000 an associate and I recorded VBA code in an Auto_Open macro in the Excel application that forwards a workbook (which is created daily) to two co-workers using Outlook email. The macro works on her PC but it causes a prompt to come up when it is executed that asks “A program is trying to automatically send e-mail on your behalf. Do you want to allow this?”

This prompt DOES NOT appear when the macro is run from any other users’ PCs. What (and where) is the Outlook or Excel setting that needs to be changed to allow the macro to run without bringing up this prompt?