What is a DKIM selector?

As many organizations tend to use multiple ESPs for their different email strategies (Marketing, Transactional, Billing, etc.), a DKIM selector is an important value that is used to identify the specific public key that is used to sign an email message. The selector is included as part of the DKIM signature, which is added to the headers of an email message. The selector allows the receiving server to locate the correct public key in the DNS record for the domain so that it can verify the authenticity of the message. Different selectors can be used for different purposes or for different parts of an organization, which allows for flexibility in managing the DKIM keys.