You can verify your domain by adding a digital signature. This confirms that you are the owner of the domain. With the DomainKeys Identified Mail (DKIM) process, your outgoing e-mails are signed using a private and public key. This allows the recipient's mail server to determine whether the e-mail really came from the sender address or if it has been forged.
The procedure described below requires that you already have an appropriate TXT or DKIM key, which you created using your Google account.