I had this problem and couldn't find anything on it on the web. Asked Bill Boswell for some help. It worked and here's his response:

Bill,
Our company has overseas customers who won't pay until they get an invoice.
Naturally, our company wants payment as soon as possible. FedEx costs $45.
So we decided to send a PDF file of the invoice in an email attachment. Our
office runs Windows 2003 and Exchange 2003. The Outlook clients are Outlook
2000. MAPI was giving us some issues, so we decided to try VBA on Outlook.
We wound up getting it to run, but we receive a Windows message from Outlook
requesting our permission to allow the application to access Outlook. A
nice security feature to be sure, but it's driving the ladies in production
control nuts. Any suggestions?
Thanks,
Tim

Tim,
There's a security management tool, the Outlook E-mail Security
Administrator Package, in the Office Resource Kit. The tool contains a dll
that's designed to handle problems like the one you're having.
The tool is discussed (kinda) in KB article 290499.
http://support.microsoft.com/view/tn.asp?kb=263556
You can download the Office Resource Kit (ORK) from
http://www.microsoft.com/office/ork/xp/default.htm (Office XP) or
http://www.microsoft.com/office/ork/2000/appndx/toolbox.htm (Office 2000).
To extract the Security Administrator Package, first install ORK on a
desktop or server. In the files it installs, find the admpack.exe file. It
should be in C:\Program Files\ORKTOOLS\ORK11\TOOLS\Outlook Administrator
Pack. (ORK10 for Outlook 2000.)
When you run admpack.exe, it deposits four files in the folder of your
choice. They are:
comdlg32.ocx
hashctl.dll
OutlookSecurity.oft
readme.doc
The readme.doc will tell you to copy hashctl.dll to the %windir%\system32
directory then register it: regsvr32 hashctl.dll
The readme.doc also tells you to do the same for comdlg32.ocx. Hold on,
though. If you're running XP on the desktop, don't copy comdlg32.ocx to the
%windir%\system32 directory. The version of comdlg32.ocx in XP works just
fine.
Now, on the machine where you installed the ORK, take a look at the
%windir%\inf folder. You should see a new set of ADM template files that
represent group policy settings for Office. The Outlook ADM file,
OUTLK10.ADM, contains a policy setting called "Outlook Virus Security
Settings." This policy setting results in a Registry entry that the
Readme.doc for the security management tool discusses:
HKCU\Software\Policies\Microsoft\Security\CheckAdminSettings
By using a GPO to apply this setting, you won't have to push out any
Registry hacks. To use the ADM template:
1. Create a new Group Policy Object (GPO) called ORK (you can call it
anything you like) and link the GPO to the OU that contains your Outlook
users.
2. Right-click the Administrative Templates object under User Configuration
and select Load Templates from the flyout menu.
3. Double-click the OUTLK10.ADM file to load the template settings.
4. In the main Group Policy Editor window, drill down to the Outlook Virus
Security Settings icon under User Configuration | Administrative Templates |
Microsoft Outlook 2002 | Tools|Options... | Security.
5. Enable this policy and check the option to Apply Individual Settings for
Outlook Virus Security.
To make this setting take effect immediately on an XP desktop, use this
command: gpupdate /force (The /force switch isn't strictly required, but I
like the sound of it.) On a Windows 2000 desktop, use this command: secedit
/refreshpolicy user_policy /enforce.
Okay, so now you're ready to apply the security setting.
1. Create a public folder in the Exchange public folder tree and call it
Outlook Security Settings. This is the required name.
2. At a client desktop that has Outlook installed, log in using an account
with Exchange administrator privileges.
3. Launch Outlook then double-click the OutlookSecurity.oft file.
4. Before the form (defined by the oft file) launches, you'll be prompted to
select a folder. Use the tree control to select the Outlook Security
Settings folder in the public folder tree.
5. The Outlook Security form is fairly complex because it deals with
programmatic forms and things like that. If you only want to send a message
without getting the popup warning, do this:
6. Select the Programmatic Settings tab.
7. On the line that starts, When Sending Items Via Outlook Object Model,
click the radio button under Automatically Approve.
8. Click Post to post the settings to the Outlook Security Settings public
folder.
9. Close Outlook.
Now, log on as an average user at the desktop where you will be running your
script to send the pdf files. Run your script and see if it works without
prompting the user.
If this doesn't work, let me know and we'll troubleshoot it together.
-bill