Recently I stumbled across a bug in the OpenDKIM configuration of the Zimbra Collaboration Server.
In ZCS 8.0.3 (Community Edition, but I guess the same applies to the Network Edition), the file /opt/zimbra/conf/opendkim.conf.in specifies the socket that OpenDKIM listens on in the following way:
This results in the following socket address of "inet6:8465@[::1]" in the final file (opendkim.conf). However, the Postfix configuration file /opt/zimbra/postfix/conf/master.cf.in specifies the socket as "inet:localhost:8465". This leads to Postfix trying to connect to an IPv4 socket, while OpenDKIM is listening on an IPv6 socket, so that the connection cannot be established.
The fix is quite easy: By changing "%%zimbraInetMode%%:8465@[%%zimbraLocalBindAddress%%]" to "inet:email@example.com" in opendkim.conf.in and restarting Zimbra, OpenDKIM can be made to listen on an IPv4 socket, so that Postfix can connect again.
The curious thing is, that this bug has already been reported half a year ago and has supposedly been fixed. However, it seems like this fix was only applied to the 9.0 branch of Zimbra and not to Zimbra 8.0.