Error When Replying to Email

Glenn Maxwell 14,226 Reputation points
2026-10-13T07:44:48.6433333+00:00

Hi Team,

One of our users is using Outlook for Mac and encounters the following error when attempting to reply to an email:

Error message: “Can't create a new message. Please make sure you have a valid certificate to send messages with S/MIME.”

We currently do not have an infrastructure in place to issue or manage S/MIME certificates.

Could you please advise how the user can reply to the email without an S/MIME certificate? Is this issue related to the original email being S/MIME-signed or encrypted, or is there a configuration in Outlook for Mac that needs to be changed?

Please recommend the appropriate troubleshooting steps or workaround to resolve this issue.

mime

Outlook | MacOS | New Outlook for Mac | For business
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  1. AI answer

    2026-10-13T07:45:14.5566667+00:00

    That error indicates Outlook for Mac is trying to use S/MIME for the reply, but no valid certificate is configured for that account.

    What this means:

    • If the original message was S/MIME-signed, Outlook may try to preserve S/MIME behavior when replying.
    • If the original message was S/MIME-encrypted, sending an encrypted reply requires certificate-based S/MIME setup.
    • Outlook for Mac requires a certificate to be added to the Mac keychain before S/MIME signing or encryption can be used.

    Since there is no S/MIME certificate infrastructure in place, the practical workaround is to avoid replying with S/MIME.

    Recommended checks and workaround:

    1. In Outlook for Mac, open a new blank email instead of using Reply.
    2. In the compose window, open See more items and check whether S/MIME is enabled.
    3. If S/MIME appears, make sure Encrypt with S/MIME and Add digital signature are not selected.
    4. If needed, go to Outlook > Accounts > select the account > Security and review whether a certificate is configured for that account.
    5. If using new Outlook, go to Settings > Mail > S/MIME and check whether automatic signing or encryption is enabled for all outgoing messages. Turn those options off if they are enabled.
    6. After disabling S/MIME options, send the response as a normal unencrypted message.

    If the reply action still forces S/MIME, use this workaround:

    • Create a new message manually.
    • Copy the recipient, subject, and any needed content from the original message.
    • Send it without S/MIME enabled.

    Relevant product behavior:

    • Outlook for Mac supports S/MIME encryption and Microsoft 365 Message Encryption as separate options.
    • S/MIME always depends on certificates.
    • In Outlook for Mac, certificates must be present in the keychain and selected in the account security settings.
    • In new Outlook, certificates are not imported automatically; they must be installed manually or deployed by policy.
    AI-generated content may be incorrect. Read our transparency notes for more information.

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  2. Kristen Tran 2,445 Reputation points Independent Advisor
    2026-10-13T08:20:01.62+00:00

    Hi Glenn,

    The AI answer above covers the main troubleshooting steps. I would just like to clarify a few points based on Microsoft’s S/MIME documentation.

    The error occurs when Outlook attempts to use S/MIME but a valid certificate is not available. Please select S/MIME Settings from the error message and make sure signing or encryption is not enabled. In the reply window, also check See more items (…) > S/MIME and make sure Add digital signature and Encrypt with S/MIME are turned off.

    As a test, create a new message to the same recipient without enabling S/MIME. If this works, you can use a new message as a workaround instead of replying directly.

    If a new message produces the same error, check Outlook > Accounts > affected account > Security. Outlook for Mac requires an appropriate certificate in the macOS keychain to use S/MIME signing or encryption.

    Please note that Microsoft’s documentation does not indicate that a reply must be signed simply because the original email was digitally signed. However, sending an S/MIME-encrypted message does require a valid certificate.

    You can find more information in Send a digitally signed or encrypted message for Mac.

    I hope the information above helps. If you have any further questions, please let me know.

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