The split domain link is bad. Try this...
Split Domain - Zimbra :: Wiki
I ended up using .forward during migration... so logically it went like this...
outside world -> sendmail -> if .forward, then ->zimbra
For this to work, I had two domains set up on my zimbra server: domain.com and zimbra.domain.com, and all users needed to have aliases for both.
Then as I migrated users, I would do this:
echo "username@zimbra.domain.com" > ~username/.forward
This is only until I feel comfortable enough to actually switch the primary MTA over.
Hope I'm not being too confusing...
-Nutz