Below is a list of the most common "selectors" used by third-party email service providers.
eversrv (OLD selector)
MailChimp / Mandrill
A DKIM selector is a string used to point to a specific DKIM public-key record in your DNS. The selector is specified as an "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.
When the receiver retrieves the public key it uses it to verify the DKIM signature.
PLEASE NOTE: No two mail service providers would have the same selector. For example, if you are sending emails from a number of different services on behalf of your domain such as Everlytic, Google, Microsoft and Sendgrid, each 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.