I just found the "Check MX Record" button on the domain in the admin console and clicked it to see what it did. Much to my surprise it told me that it had failed. I know that the DNS records are correct so I clicked on the details to see what it was finding and saw this:
Code:
Domain is configured to use SMTP host: email.pyxislab.com. None of the MX records match this name.
Found the following MX records:
0 email.pyxislab.com.
Anybody have a clue as to how those are different at all?

Code:
zimbra@email:~$ zmprov gd pyxislab.com | grep -i publicservicehostname
zimbraPublicServiceHostname: email.pyxislab.com
zimbra@email:~$ dig +short MX pyxislab.com
0 email.pyxislab.com.
(I also tried with "zimbraPublicServiceHostname: email.pyxislab.com.")

From what I can tell this button just doesn't work...which isn't much help to people who might actually have issues.