When I log in to the admin page of my Zimbra server I have the same error with the tab 'Message Count', 'Message Volume' and 'Anti-Spam/Ati-Virus Activity":

This is not an MTA and will not have any MTA statistics.

Never seen this message before?

Some extra information:
Release 6.0.7_GA_2473.RHEL5_20100616214455 CentOS5 FOSS edition.

$ zmcontrol status
antispam Running
antivirus Running
ldap Running
logger Running
mailbox Running
mta Running
snmp Running
spell Running
stats Running