When you receive a postal letter, the return address in the top left corner of the envelope tells you where and who it is from. But, the sender can write any name and address. There is no guarantee that the contents of the envelope are from the sender identified in the top left corner of the envelope. Emails are not different.
Spoofing is the word for impersonating an email sender’s identity. It is forgery. If you are able to read email headers, many times you can identify the email address of the sender. Sometimes you can’t when the spoofer connects directly to an email server. The headers of these emails show that the email was sent from an account owner’s email server, not the email server of the spoofer.
SMTP (Simple Mail Transfer Protocol) is used to send outgoing email. SMTP does not require authentication of the sender. As email account holders, we do not have control or access to the email server sending our emails.