DKIM helps verify the sender and integrity of a message.
It allows an email system to prove that a message was not altered during transit (meaning it is not forged), and that the message came from the specified domain.
You can use the following steps to enable DomainKeys:
1. Login to your account for Plesk,
2. Manage the domain you'd like to add the DomainKey for,
3. Click the "Mail" tab.
4. Now that you're on the Mail page, click the "Mail Settings" tab to access your accounts mail preferences.
5. Under the "Mail Settings" area you should make sure that the option to enable "Use DomainKeys spam protection..." is checked. If not, check it and click "OK".
6. Now go to "Websites and Domains" tab, click the "DNS Settings",
7. We need to add a new DNS record so click the "Add Record" button,
8. The type of record we're creating is a "TXT" record so choose that from the dropdown
9. Enter in your TXT record details,
That's it! You should be able to authenticate your domain.