DKIM (stands for DomainKeys Identified Mail) is an E-mail authentication method which allows for a unique Signature to be added for each E-mail message you send. The Signature is generated by using encryption with public and private keys. The receiving mail server can use the public key to determine if your private key was used to generate the message's Signature, and that the message body and the most important headers were not altered during the transit.
To specify Domain DKIM settings, use the WebAdmin Interface to open the Domain Settings pages. Open the Security page and click the DKIM link.
To enable adding the Signatures to outgoing messages you need to complete the following steps:
Generate (or import externally-generated) Private Key in the same way as assigning a Private Key for the Domain. After the Private Key is assigned the corresponding Public Key will be displayed.
Specify the value for Domain it should be either the current Domain name, or its "organizational domain" name.
Choose and specify the value for Selector an arbitrary string.
Create DNS TXT record for name Selector._domainkey.Domain with value "v=DKIM1; p=MIGfMA0GCSqG..." where the value of "p=" is your Public Key.
Use Check button to verify that the DNS record exists and contains the right Public Key. Remember that newly created DNS records may need to propagate through the world.
After all the above steps are complete, switch the Enabled to Yes.
It is recommended to rotate the Private Key regularly (about every 3 months), by generating new Keys and creating new DNS record for a new Selector.