About Domain-based Message Authentication (DKIM)
DomainKeys Identified Mail (DKIM) is a content verification mechanism for email. It detects if a message has been modified after it was sent. DKIM is intended to prevent forged sender addresses in emails (the "from" line) by validating an email claiming to be sent from a specific domain was actually authorized by the domain.
DKIM to evaluate whether or not a message should be trusted, and it's a critical step in protecting recipients from phishing. DKIM DNS records are tied to a specific "selector" in the DNS(a domain can have multiple selectors) and a unique encryption token. If you are unsure what your selector is, try using the "Email Health Report" to identify your configuration.