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 -s mail -d example.com

Here, "-s mail" indicates that we are using "mail" for the selector name. 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 mail.txt file:

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

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