I tried upgrading from 4.5.6 to 4.5.11 then to 5.0.2
I had the same mta password request.
Then I found this comment about sunsets:
"You MUST update the server OS on 4.5.x before you begin the upgrade to ZCS 5.0. To do this you must first upgrade from 4.5.x to 4.5.10 (which has the current server OS) and THEN you can safely run an upgrade to ZCS 5.0. Not doing so will fail the install."
I found this comment on
http://www.zimbra.com/products/sunset_binaries.html
I found a link to this page called "Important information about Sunset binaries." on this page
http://www.zimbra.com/community/downloads.html
Can you see the link?
Going from 4.5.6 to 4.5.10 then to 5.0.2 gave me the same 'password' request from mta.
Chris