sending server publishe public key in DNS
sending server use private key to sign a singuare to email send to receving server
receving server grabs public key from sending server's DNS
use public key to verify sending server signature
https://www.fastcomet.com/kb/enable-dkim-and-spf-records?psafe_param=1&utm_id=gy.search.usca&utm_source=google&utm_medium=cpc&utm_campaign=21034175404_159447262216&utm_term=g_dsa-391668958313__&utm_content=691145754903&locationid=9001529&device=c_c&gad_source=1&gclid=EAIaIQobChMIv_KWgdnNiAMVag6tBh3o8A26EAAYAyAAEgIoLvD_BwE
