Although OpenDKIM is optional, I highly recommended you use it unless you want your mail to end up in someone's spam folder. See the README in `opendkim` folder for more info on setting it up.
You should also add PTR record to your IP (aka Reverse DNS) which is done by your server provider.
And finally, generate and add an SPF record to your domain, search for instructions on this - there are a few generator site around and the setup steps depend on your domain name provider.