After some further research, I found a fix for this, on the following webpage.
Unable to sign Outlook 2010 VBAProject
The fix is to add an extra step (underlined below).
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- Create certificate "MyName" using selfcert.exe.
- Export certificate "MyName" from personal store to .cer file.
- Import certificate "MyName" from .cer file to trusted store.
- Go to Outlook > Developer > Visual Basic > Tools > Digital Signature > Sign As; click on Choose button. At this point, the certificate I created is displayed (there is no option to specify a store or certificate; the one I created is displayed automatically), and when I click on the link "Click here to view certificate properties", it appears to be the trusted one ("This certificate is intended for the following purposes: Ensures software came from software publisher ... ", etc.).
- Click on OK. It then says "The VBA project is currently signed as Certificate Name: MyName"
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Click on OK, then click on the Save icon (or File > Save, or Ctrl + S). (If this step is skipped, the digital signature is not saved.)
- Go to Outlook > Developer > Macro Security and select option "Notifications for digitally signed macros, all other macros disabled."
- Click on OK, close VBA, close Outlook, click on "Yes" when prompted "Do you want to save the VBA project 'VbaProject.OTM'?".
- Re-open Outlook.
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Now the macro will run without any security prompts.