What is a DKIM signature?

DomainKeys Identified Mail (DKIM) allows senders to associate a domain name with an email message, thus vouching for its authenticity. This is done by "signing" the email with a digital signature, a field that is added to the message's header.

What is a SPF record?

An SPF record is a type of Domain Name Service (DNS) record that identifies which mail servers are permitted to send email on behalf of your domain. The purpose of an SPF record is to prevent spammers from sending messages with forged From addresses at your domain.