The private key extraction procedure given at
Commercial Certificates - Zimbra :: Wiki
no longer works.

D. Extract the private key for use with postfix, perdition, ldap

# /opt/zimbra/java/bin/javac ExportPriv.java
ExportPriv.java:5: warning: sun.misc.BASE64Encoder is Sun proprietary API and may be removed in a future release
import sun.misc.BASE64Encoder;
^
ExportPriv.java:32: warning: sun.misc.BASE64Encoder is Sun proprietary API and may be removed in a future release
BASE64Encoder myB64 = new BASE64Encoder();
^
ExportPriv.java:32: warning: sun.misc.BASE64Encoder is Sun proprietary API and may be removed in a future release
BASE64Encoder myB64 = new BASE64Encoder();
^
3 warnings

# /opt/zimbra/java/bin/java ExportPriv /opt/zimbra/ssl/ssl/commercial.keystore tomcat zimbra > /opt/zimbra/ssl/ssl/commercial.key
Exception in thread "main" java.lang.NoClassDefFoundError: ExportPriv

At this point /opt/zimbra/ssl/ssl/commercial.key is empty.