[zimbra@mail ~]$ zmcontrol -v
Release 8.6.0_GA_1153.RHEL7_64_20141215151110 RHEL7_64 FOSS edition, Patch 8.6.0_P2.
[zimbra@mail ~]$ /opt/zimbra/bin/zmcertmgr createcsr comm -new -keysize 2048 -subject "/C=ID/ST=Jabar/L=Bandung/O=rgiapratama/OU=IT/CN=mail.rgiapratama.net" -subjectAltNames "mail.rgiapratama.net"
[zimbra@mail ~]$ vi /opt/zimbra/ssl/zimbra/commercial/commercial.csr

Submit commercial.csr to Comodo, after approval process you’ll get email from Comodo with your EssentialSSL Certificate in zip file. After download, upload zip file to your mailserver and extract it.

[zimbra@mail ~]$ unzip mail_rgiapratama_net.zip
[zimbra@mail ~]$ cat AddTrustExternalCARoot.crt COMODORSAAddTrustCA.crt COMODORSADomainValidationSecureServerCA.crt > ca_chain.crt
[zimbra@mail ~]$ /opt/zimbra/bin/zmcertmgr verifycrt comm /opt/zimbra/ssl/zimbra/commercial/commercial.key mail_rgiapratama_net.crt ca_chain.crt
** Verifying /opt/zimbra/mail_rgiapratama_net.crt against /opt/zimbra/ssl/zimbra/commercial/commercial.key
Certificate (/opt/zimbra/mail_rgiapratama_net.crt) and private key (/opt/zimbra/ssl/zimbra/commercial/commercial.key) match.
Valid Certificate: /opt/zimbra/mail_rgiapratama_net.crt: OK
[zimbra@mail ~]$ /opt/zimbra/bin/zmcertmgr deploycrt comm mail_rgiapratama_net.crt ca_chain.crt
[zimbra@mail ~]$ /opt/zimbra/bin/zmcertmgr viewdeployedcrt
[zimbra@mail ~]$ service zimbra stop
[zimbra@mail ~]$ /opt/zimbra/bin/zmcertmgr addcacert /opt/zimbra/ssl/zimbra/commercial/commercial.crt
[zimbra@mail ~]$ service zimbra start