This is what I did to fix this:
I used info in artturi's post 37801
This will then give a final result in postfix/conf/main.cf :
POSTCONF virtual_alias_maps LOCAL postfix_virtual_alias_maps
Now I edited the /opt/zimbra/postfix/conf/virtual and added at the bottom of that file:
virtual_alias_maps = ldap:/opt/zimbra/conf/ldap-vam.cf ,
Now I run the following command to apply the mail forward and restarted postfix:
Then I did a test by sending email from my gmail account to email@example.com and it was delivered to firstname.lastname@example.org.
Other way of doing this is of course to set up a distribution list in the Zimbra GUI but I guess that gets very quickly very loaded and using my way I can comment all the setup and get better overview of things.
My only concern is if this will all be defaulted again in next upgrade? Is there some other better way to do this? Pls. comment.