SMIME ZWC Certificates

S/MIME certificates

   KB 24164        Last updated on 2022-06-7  

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S/MIME stands for Secure/Multipurpose Internet Mail Extensions. It is a standards-based method of public/private key encryption. S/MIME is based on asymmetric cryptography it is commonly used for email and MIME data. S/MIME enables email security features by providing encryption, authentication, message integrity and other related services. It ensures that an email message is sent by a legitimate sender and provides encryption for incoming and outgoing messages. To enable S/MIME based communication, the sender and receiver must be integrated with public key and signature issued from a certificate authority (CA).

We're not going to go into great detail on what S/MIME is here, so please feel free to read the Wikipedia article for more background.

These are the following ways to get a SIME/certificate.

1. Buy a S/MIME certificate from authority like Comodo, SSLshoper , digicert etc.

2. There are few companies which provides S/MIME free for a year or 30 days.

3. Use self-signed S/MIME certificate.

Here are the steps to generate a self-singed SMIME certificate:

  • Generate a key and set password which will be needed later during CSR generation.
 openssl genrsa -des3 -out cert.key 4096
  • Generate the CSR, use your email ID as Common Name while generating CSR and don’t set “A challenge passwordon” CSR and leave it blank, just press enter.
 openssl req -new -key cert.key -out cert.csr 
  • Create the certificate key.
 openssl x509 -req -days 365 -in cert.csr -signkey cert.key -out cert.crt
  • Convert certificate into .p12. Set “Export Password” which will be used during the upload of S/MIME to Zimbra web client.
 openssl pkcs12 -export -in cert.crt -inkey cert.key -name "Your Name" -out cert.p12

Note: In case of self-signed S/MIME certificate, OCSP check should be disabled for a S/MIME certificate. (OCSP) is an Internet protocol used for obtaining the revocation status of an X.509 digital certificate. The full form is Online Certificate Status Protocol (OCSP).

 su - zimbra
 zmprov mcf zimbraSmimeOCSPEnabled FALSE
 zmpro fc all

Importing certificate into the Zimbra Web Client (ZWC). Using free S/MIME certificate which was opted from ACTALIS for a demo.

1. Be sure, ZCS is installed with Network Edition version of the product and license includes S/MIME users.

 su – zimbra
 zmlicense -p | grep SMIMEAccountsLimit

2. Check the Zimlet (com_zimbra_secureemail), it should be enabled on a user or COS level to use S/MIME with ZWC.


Note: Disable com_zimbra_smime zimlet which is an old zimlet not used anymore.

3. Enable SMIME feature from CLI or GUI. Taking two users in lab to show you SMIME working.

 zmprov ma zimbraFeatureSMIMEEnabled TRUE
 zmprov mc <COS NAME> zimbraFeatureSMIMEEnabled TRUE


 zmprov ma zimbraFeatureSMIMEEnabled TRUE
 zmprov ma zimbraFeatureSMIMEEnabled TRUE

4. Upload the S/MIME certificate into ZWC of a user, which was purchased from any SSL provider.

Presences > Secure Email > Browse to certificate

Note: It will ask the password of your SMIME certificate.

Zimbra-smime-02.png Zimbra-smime-03.png Zimbra-smime-04.png

Note: Same steps have to be followed for an another user to upload a S/MIME certificate.

Exchange public key for sending encrypted email.

Both users need to send a singed email to each other to share a public certificate.

1. Sending a signed email from one user to other.


2. Reply back from other user with a singed email.


Note: Now both users have exchanged their public key with each other and ready to send an encrypted email.

Sending an encrypted email from one user to other user.

1. Compose an email and select “Sign and Encrypt” before sending.


2. Other user is able to view the email with singed and encrypted icon.



  • If the following error appears in the web client while importing a S/MIME into ZWC and below Exception appears in the mailbox.log, that means either Intermediate certificate of S/MIME was not added to java keystore on the mailbox server or S/MIME is revoked not verified by CA authority.


       at com.zimbra.cs.service.smime.SmimeServiceException.CERT_VALIDATION_FAILED(
       at com.zimbra.cs.smime.SmimeCryptoUtil.validateCertificate(
       at com.zimbra.cs.service.smime.SaveSmimeCertificate.handle(
       at org.eclipse.jetty.util.thread.QueuedThreadPool.runJob(
       at org.eclipse.jetty.util.thread.QueuedThreadPool$
       at java.base/
Caused by: Path does not chain with any of the trust anchors

Solution: Either get Intermediate of your S/MIME and add to the java key store or disable OCSP check.

 su - zimbra
 zmcertmgr addcacert /tmp/<Intermediate certificate>      
 zmmailboxdctl restart

To disable an OCPS check:

 su - zimbra
 zmprov mcf zimbraSmimeOCSPEnabled FALSE
 zmprov fc all
  • The following java error appears if an old zimlet (com_zimbra_smime) is enabled which is not supported.


Solution: Disable the old zimlet (com_zimbra_smime) and enable the new zimlet (com_zimbra_secure).

Submitted by: Prabhat Kumar
Verified Against: ZCS 9.0, 8.8, 8.7, Date Created: 2017-08-23
Article ID: Date Modified: 2022-06-07

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