Office 365 / Exchange Online - DKIM Config
exchange-admin-dkim

TL;DR
If your organization uses a custom domain with Office 365, configuring DomainKeys Identified Mail (DKIM) is necessary to digitally verify that your outgoing emails are authorized and have not been modified in transit.
- DKIM verifies email authenticity, which helps minimize the risk of receiving spam and phishing scams.
- By default, DKIM is enabled for the `onmicrosoft` domain but requires manual setup for your primary custom domain.
- Enabling DKIM necessitates adding specific CNAME records to your external domain DNS management portal.
- The setup process must begin by accessing the DKIM protection settings within the Exchange Admin portal.