Generating DKIM Key

First, install the package containing opendkim-genkey. On Debian/Ubuntu:

apt-get install opendkim-tools

Now, generate the keypair (replace example.com with your domain name):

opendkim-genkey -t -s mail -d example.com

Here, "-s mail" indicates that we are using "mail" for the selector name, and "-t" indicates we will run DKIM in test mode; most DKIM users opt for test mode, but you can remove the "-t" flag if desired, in this case mail servers verifying DKIM will reject mail that isn't signed.

The command will generate a public key in mail.txt and private key in mail.private.

Create a DNS TXT record at "mail._domainkey.example.com." based on the public key:

mail._domainkey.example.com. IN TXT "k=rsa; p=PpYHdE2tevfEpvL1Tk2dDYv0pF28/f 5MxU83x/0bsn4R4p7waPaz1IbOGs/6bm5QIDAQAB"

Finally, copy the mail.key contents to the private key text area.