Email delivery is based on SMTP protocol, which was designed initially around 1982 and defined inside RFC821.
At these early days nobody did think about security and so the initial SMTP protocol is missing any security features.
Today, the spam industry profits from such a poor SMTP implementation and malicious messages like phishing and scam are a threat for a secure private and business email traffic.
As an active email sender or email server manager, you are certainly aware of the importantance to follow email best practices in order to get your messages delivered.
One of these best practices is to ensure that you’re properly authenticating your email messages.
Fortunately, there are several email authentication techniques like SPF and DKIM.