Microsoft Outlook: Correctly signed e-mails (S/MIME) are displayed as invalid after the signing certificate expires
Assume the following scenario:
- A user has received an email message in the past.
- The message was signed with an S/MIME certificate.
- The sender's signing certificate was issued by a certification authority that is trusted by the recipient.
- Thus, the signature was recognized as valid at the time the message was received.
- The user opens the mail again some time later and finds that the signature is classified as invalid.