I just wanted to share my experience here afor Zimbra.
I use Zimbra latest NE on Centos 5.5 32-bit.
I did not succedded to make it work under heartbeat and drbd but i did it with cold stand by, instructions that i found here :Harley77-Backup - Zimbra :: Wiki
2nd method work very good for me.
I have the stand by server with same hostname as live one. I do rsync both servers every 10 minutes via cross over cable. It takes just 1-2 minutes to rsync more than 50GB of mails.
I am connecting clients with a record and not ip so that once live server goes down i just fire up zimbra on cold stand by server and change a record ip on my DNS.
By fire up zimbra on cold stand by server i did automatically coldstandby file on /root/ as per instuctions, to be deleted so that server not re rync with live server and like this to lose mails.
This is the best solution i have found for zimbra except clustering for redudancy