When it comes to your email address, you might presume that only you can send an email that originates from that address. Similarly, when you receive an email, you might presume that whoever owns the email address it claims to originate from has sanctioned that message. However, there is nothing in the original standards for email that guarantee either of these things. In reality, this aspect of electronic mail works very similarly to physical mail. A sender address might be on the envelope, but that alone proves nothing.
Of course, as email grew in popularity, solutions were devised to address this problem, but since they are not part of the core standards, they remain optional. It requires careful configuration on both the sender’s and receiver’s side to make it possible to verify the authenticity of sender information.