This article explains how you can digitally sign a file or a macro project by using a certificate. If you don't already have a digital certificate, you must obtain one. To test macro projects on your own computer, you can create your own self-signing certificate by using the Selfcert.exe tool.
Useful for future reference. I'm playing with a bit of VBA in Outlook, and found that my code would work fine right after writing it, but if I restart Outlook, the macros will be disabled unless I first sign them as described in this article.