Things to check I found here on the forums for you:
- SElinux should be disabled
- Check the permissions of your postfix conf folder
Code:
ls -l /opt/zimbra/postfix/conf
total 208
-rw-r--r-- 1 root postfix 15647 2007-03-28 22:55 access
-rw-r--r-- 1 root postfix 8240 2007-03-28 22:55 aliases
-rw-r--r-- 1 root postfix 10947 2007-03-28 22:55 canonical
-rw-r--r-- 1 root postfix 9622 2007-03-28 22:55 generic
-rw-r--r-- 1 root postfix 16393 2007-03-28 22:55 header_checks
-rw-r--r-- 1 root postfix 11942 2007-03-28 22:55 LICENSE
-rw-r--r-- 1 zimbra zimbra 1781 2007-08-01 23:48 main.cf
-rw-r--r-- 1 root postfix 14379 2007-03-28 22:55 main.cf.default
-rw-r--r-- 1 root postfix 1783 2007-03-28 22:55 makedefs.out
-rw-r--r-- 1 root postfix 5116 2007-07-06 23:24 master.cf
-rw-r--r-- 1 root postfix 17077 2007-03-28 22:55 postfix-files
-rwxr-xr-x 1 root postfix 6366 2007-03-28 22:55 postfix-script
-rwxr-xr-x 1 root postfix 22018 2007-03-28 22:55 post-install
-rw-r--r-- 1 root postfix 1024 2007-08-02 00:53 prng_exch
-rw-r--r-- 1 root postfix 6523 2007-03-28 22:55 relocated
-rw-r--r-- 1 root postfix 1629 2007-03-28 22:55 TLS_LICENSE
-rw-r--r-- 1 root postfix 11033 2007-03-28 22:55 transport
-rw-r--r-- 1 root postfix 11732 2007-03-28 22:55 virtual
- Check if you have a Firewall that blocks it
If it is about the permissionssearch this forum for 'fix permissions'