SPF, DKIM and DMARC

Three DNS records that tell receiving servers your mail is genuinely yours. Without all three, a significant share of your email is filtered before any content check happens — and since Google and Yahoo tightened bulk sender requirements, authentication is a prerequisite rather than an optimisation.

Set them up before sending anything. Debugging them after a domain has been damaged is much slower.

SPF

A TXT record on the domain listing who may send for it.

v=spf1 include:_spf.google.com include:sendgrid.net ~all