Also, I would suggest voting for this bug/rfe I created last week - to move the "Notify Zimbra" question to the very last thing in the install/upgrade script.
My situation was just that during an upgrade I got called away to look at something else, and when I returned, the mail system was still down, with the script sitting there asking for permission to notify zimbra. I see no reason this shouldn't be done after everything is back up and running.
https://bugzilla.zimbra.com/show_bug.cgi?id=57543