Microsoft Outlook cannot sign or encrypt this message because your certificate is not valid

Question

One of my employees erroneously sent out an email to one of my districts that everyone needed to have a certificate installed for their Outlook client. This error was not caught until about an hour later, and a few employees had already installed this certificate that was meant for senior leadership only.

After revoking the certificates, unchecking the Add digital signature to outgoing messages and Send clear text signed message when sending signed messages boxes, and publishing to GAL, these employees are getting the error in the title when attempting to respond to any employee with a certificate.

I noticed that whenever these employees, who temporarily had certificates which were later removed, reply to certificate owners, they have the Sign option selected by default.

Any idea on how to prevent that option from being selected by default or otherwise rectify this issue?

Solution

Try this procedure for the affected employees :

1. Open Microsoft Outlook
2. Click on the File tab
3. Click on the Options tab
4. Click on the Trust Center tab
5. Click on the Trust Center Settings… button
6. Click on the E-mail Security tab
7. Under the heading Encrypted e-mail, uncheck the checkbox that says
   Add digital signature to outgoing messages
8. Click the OK button

Comments

[aLearningLady]

Thanks, harrymc, but I've already done that and the user is still experiencing this issue. Note that this issue is only when replying to emails that have been signed by certificate owners

[harrymc]

Seems to be a known gotcha but without a published solution. Try the registry fix in this article. If no go, compare the registry entries under the registry key HKEY_CURRENT_USER\SOFTWARE\Microsoft\Office\15.0\Outlook\Sec‌​urity between computers that have/have-not this problem.