I see a lot of this kind of problem, but with the "Address already in use" error and not the "Permission Denied" error I am getting.
I installed OpenDKIM via this blog:
http://blog.matoski.com/articles/spf-dk-dkim-plesk-debian/
And everything went swimmingly until I restarted Postfix and my log immediately blasts with:
OpenDKIM Filter: Unable to bind to port inet:8891@localhost: Permission denied
OpenDKIM Filter: Unable to create listening socket on conn inet:8891@localhost
smfi_opensocket() failed
I have verified that nothing else is listening on that port and I have gone back through the blog post and can confirm that everything else is correct.
[root@server opendkim]# ps aux | grep opendkim
root 18173 0.0 0.0 103252 864 pts/0 S+ 18:39 0:00 grep opendkim
[root@server opendkim]#
[root@server opendkim]# netstat -nlp | grep 8891
[root@server opendkim]#
I suspect this is something to do with selinux stopping this from creating another listener, however I do not know how to change that. Or, perhaps I'm way off base.