See this article from Microsoft about digital signatures and security for macros.

From Outlook-Macros.blogspot.com for self signing a certificate.

To enable VB (Visual Basic) macros to run in Outlook with high security level, you need to self sign a certificate. (Code signing certificates cost a few hundred dollars a year.) To self sign, use selfcert.exe.

1. Run C:\Program Files\Microsoft Office\OFFICExx\selfcert.exe (http://www.howto-outlook.com/howto/selfcert.htm)
2. In the VB Editor go to Tools, Digital Signature and add the certificate to the project.
3. Get out of the VB Editor and Outlook and go back in. Outlook will ask if you want to always trust macros from this certificate. Say yes.

Outlook macro project is here.
File: VbaProject.OTM
Folder: %appdata%\Microsoft\Outlook
Example: C:\Documents and Settings\username\Application Data\Microsoft\Outlook

To Deploy Outlook Macros with Self-signed Certificate

The above works on your development computer, but when you want to install the code on another computer you need some extra steps.

If the other computer is going to share the same macros, you can copy the VbaProject.OTM file to the other computer.

In Outlook, if you are running with high security as you should be, you will get an error message with a security warning about macros. You will get this error message either when you start Outlook or you may need to go to Outlook, Tools, Macro, Visual basic Editor to get the message.

The always trust macros from this publisher checkbox will be grayed out as well as the Enable Maros button. Click the Details button.

You will get a Certificate Error message - This CA Root certificate is not trusted. To enable trust, install this certificate in the Trusted Root Certificate Authoritites store.

If the details are you or a trusted source (note that self signed certificates can be forged) then click the Install Certificate button at the bottom and follow the steps to install.

You are presented with two options. The first option is to let Windows automatically choose the store. Choose the second option and click Browse. Then select the Trusted Root store.

After completing the certificate installation, exit Outlook and go back in.

When you get back into Outlook and get the security warning again, you will be able to check the box to trust this publisher and click the button to Enable Macros.