Error When Replying to Email

Question

Error When Replying to Email

Glenn Maxwell 14,226

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

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2 answers

Your answer

Answer 1

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.

Kristen Tran 2,445 Reputation points Independent Advisor

2026-10-14T11:22:18.02+00:00

Hi Glenn,

Just checking in to see if you've reviewed the proposed answer. Any update would be appreciated.

Answer 2

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:

In Outlook for Mac, open a new blank email instead of using Reply.
In the compose window, open See more items and check whether S/MIME is enabled.
If S/MIME appears, make sure Encrypt with S/MIME and Add digital signature are not selected.
If needed, go to Outlook > Accounts > select the account > Security and review whether a certificate is configured for that account.
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.
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.