Can start with the smtp standard https://tools.ietf.org/rfc/rfc5321.txt https://tools.ietf.org/rfc/rfc5321.txt and follow the citations at the bottom to learn m

Can start with the smtp standard https://tools.ietf.org/rfc/rfc5321.txt https://tools.ietf.org/rfc/rfc5321.txt and follow the citations at the bottom to learn more about DKIM ect. OpenSMTPD has a few presentations on the changes they made and explains architecture https://www.opensmtpd.org/papers.html https://www.opensmtpd.org/papers.html