What is mail spoofing?
Email spoofing the is act of sending messages via email, faking the sender email address. This technique is commonly used for spam and phishing. The authors use it to mask their real identity and protect themselves from being reported, or to gain the trust of the recipient.
One example is for a user to pretend to send messages on behalf of a popular bank or other financial institution. They will send multiple emails telling customers to log in their online accounts and take some kind of action. The login link from the email will also be fake – leading to a page similar, but not identical to the login page of that bank. The recipient, if not careful, may try to login and thus submit their username and password to the falsifier.
Can I stop them from sending unauthorized emails on my behalf?
You can use SPF as a counter measure against spoofing. Sender Policy Framework (SPF) is an email validation system created for that very purpose.
Click on the link to learn how to create an SPF-record.