Hello YMTBMSETC,
From your post, my understanding is, you required a third-party certificate for your current account, installed it on local PC and imported to Outlook to sign or encrypt your outgoing messages from Outlook desktop client. And it worked well before this issue occured.
If so, please try to update Outlook desktop client as some client issue can be fixed via updating. Besides, please refer to following steps to check whether this certificate is still valid (PC running Windows):
1. Open control panel, navigate to Network and Internet-> Internet Options.
2. In prompt Internet Properties Window, select “Content” tab->Click on “Certificates” button.
3. In prompt Certificate window, select “Personal” tab->Select the certificate you applied for and issued to your current account.
4. Please check its valid time and make sure the address it issued to fully match your current account address:
If your PC isn’t running Windows OS, you may contact related provider to find where your certificate is installed and check its values.
Meanwhile, may I collect following information from you for checking?
1. Regarding “some of my emails are coming back undeliverable”, it seems to be non-delivery report, if so, could you please share it with me in Private Message for checking: https://answers.microsoft.com/en-us/privatemessage/inbox.
2. Please also share a full screenshot of error message "Microsoft outlook cannot sign or encrypt this message because your certificate is not valid".
3. Office suite (is it Office 365 ProPlus) and its full version number.
Best Regards,
Anna
