Hi @johnk ,
Your Digital ID name cannot be found by the underlying security system
Did this issue occur with a specific user on speicific client only? Could it be reproduced on other outlook client? Could it work normally if you send an encrypted message to others in Outlook?
For the error itself, I have researched a lot about it and found the issue may be related to your certificate configuration, in order to first confirm if you have configured your certificate successfully and choose the right setting(File>Options>Trust center>Trust center settings>Email security)
If you have checked that and the issue still exists there, based on my research, I found a related thread, which mentioned some detailed steps to re-configure your certificate and many users told it was helpful to them, please kindly check if Nathan James Heinz's post could also work for you: Your Digital Id Name Cannot Be Found By The Underlying Security.
Hope that would be helpful to you.
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