Self-Signed Certificate on your Zimbra server is expired? Follow the instructions given below, and you will be back online asap.
1. Begin by generating a new Certificate Authority (CA).
/opt/zimbra/bin/zmcertmgr createca -new
2. Then generate a certificate signed by the CA that expires in 365 days.
/opt/zimbra/bin/zmcertmgr createcrt -new -days 365
3. Next deploy the certificate.
/opt/zimbra/bin/zmcertmgr deploycrt self
4. Next deploy the CA.
5. To finish, verify the certificate was deployed to all the services.
Resetart your zimbra and you will be good.
I have this tested against Zimbra 8.