Thanks for your reply. I followed the instructions on your linked webpage successfully. (A couple of "Next" and "OK" confirmation steps were not mentioned, but I clicked through them OK.)

In Outlook, I repeated the process of choosing the (now trusted) certificate, closing Outlook, and saving the VBA project when prompted.

However, when I re-open Outlook, the digital certificate setup still says "The VBA project is currently signed in as Certificate Name: [No certificate]", and when I attempt to run the macro, it says "The macros in this project are disabled....". It appears the certificate selection is not being saved.

(I originally followed this set of instructions ...
Signing your own macros with SelfCert.exe - HowTo-Outlook
... but I am not getting the prompt mentioned in the last step. ("Since it is the first time that you are using the certificate, you’ll be prompted what to do. Select that you’ll always trust the macros or documents from this publisher and you’re done!")

How can I fix this?