A DKIM selector is a string used to to point to a specific DKIM public key record in your DNS. It is specified as "s=" tag in the DKIM-Signature header field, and can be found in the technical headers of an email.

Validation on the receiver side uses the selector in combination with the signing domain in order to carry out a DNS query and find the public key in your DNS. 

For example:

“selector”._domainkey.yourdomain

When the receiver retrieves the public key it uses it to verify the DKIM signature.

NOTE: No two services should use the same selector. If you are for example sending emails from a number of services on behalf of your domain such as: Google, Mailchimp, Salesforce and Sendgrid, each and every one must use a unique key and selector in your DNS. If the selectors were the same the recipient would not be able to tell which key to use in order to decipher a particular email. 

OnDMARC’s free tool Investigate lets you verify your DKIM set up to ensure you authenticate your emails correctly.

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