Email signature verification

How to verify an S/MIME signature

If you received a digitally signed email, your mail app should indicate that the message is signed. Use the client-specific steps below to confirm the signature is valid and inspect the certificate details.

Choose your email client

Select the app you use to see what to look for.

  1. Open the signed message details

    Open the email and click on the orange signature icon in the message header to bring up Outlook's security details.

    Outlook message view showing the signed-message control in the header.

  2. Verify the certificate and sender identity

    In the details panel, confirm that Outlook reports a valid digital signature and that the certificate belongs to the sender you expect.

    Outlook security details panel showing the digital signature status and sender certificate.

  1. Click the signature symbol in the top right

    Open the email and click the small S/MIME signature icon next to the message time in the upper-right corner.

    Thunderbird message view showing the S/MIME signature icon in the top right.

  2. Check the full sender name in the popup

    Thunderbird will open a security popup. Verify that the message is signed and that the full sender name is the expected one.

    Thunderbird security popup showing the signed status and the full sender name.

  1. Open the message details

    Open the email and click the small dropdown in the message header so Gmail shows the full sender and delivery details.

    Gmail message header with the details dropdown highlighted.

  2. Check that Gmail marks the message as signed

    In the expanded details, check the verification status (green checkmark) and click on "Sender info".

    Expanded Gmail message details showing the signed status.

  3. Open the certificate details and verify the sender

    Check that the certificate identity matches the sender you expect.

    Gmail signer details panel showing the certificate identity.

  1. Find the signed sender indicator in the header

    Open the message and look below the sender name for the signed indicator next to the sender address.

    Apple Mail message header with the signed sender indicator highlighted.

  2. Click the signed indicator to open signature details

    Select the signed sender badge and choose Show Details to open Apple Mail's signature information popup.

    Apple Mail signed message popup with the Show Details option.

  3. Confirm the authenticated sender address

    In the confirmation window, verify that Apple Mail marks the message as authenticated and that the sender address matches the person you expect.

    Apple Mail authenticated message dialog showing the confirmed sender address.

Using another email client?

Look for wording such as digitally signed, signature is valid, or a certificate viewer linked from the message header. If your client exposes certificate details, confirm that the signer identity matches the expected sender address and that the certificate chain is trusted.