How to use DKIM with your custom domain in Office 365
When you enable DomainKeys Identified Mail (DKIM), you authorize a domain to associate, or sign, its name to an email message by using cryptographic authentication. Email systems use this digital signature to help determine if incoming email that they receive is legitimate.
DKIM works with SPF and DMARC to authenticate mail senders and ensure that destination email systems trust messages sent from your domain. While you can use SPF alone or SPF and DMARC without DKIM, implementing DKIM provides additional protection against spoofing and phishing email