DKIM

Your email provider signs each message with a private key. You publish the matching public key as a record in your domain's DNS settings. The inbox reads that record and checks the signature. DKIM works together with SPF and DMARC.

Gmail asks every sender to set up SPF or DKIM, and bulk senders to set up both. Set up all three before you send any cold email. Your email provider's help pages give the exact record to add. Then warm up the domain.