DKIM, DMARC and SPF are tricky to get right. Alex Blackie's "Email Authenticity 101: DKIM, DMARC, and SPF" is a great overview of best practices around these technologies. Article Archive

There's a really good tool to check your working setup by sending an email and checking each of these headers. console

Fastmail allows you to open mailto:// links from your browser. Guide

Prevent a domain from being used for email fraud by hardening it. Article Archive