Upgraded from which version to which version? Which operating system? What errors, if any, are in the logs? What happens when you run zmprov? Are the zimbra services running i.e. run zmcontrol status.
What's the output of the following run on the zimbra server:
Code:
host `hostname` <-- type as-is with backticks not single quotes
dig yourdomain.com any
dig yourdomain.com mx