Sender spoofing is the practice of forging an email address or display name so a message appears to come from someone else. The visible sender can be made to look like a coworker, a vendor, or an internal system, even when the message was sent from a different server or account.
In cyber security, sender spoofing matters because humans and filters often trust the header that names the sender. Attackers use it to increase opening rates, hide the real origin of malware, and make phishing messages harder to trace. It is common in mass-mailing worms, business email compromise, and reply-chain fraud. Defenders reduce the risk with SPF, DKIM, and DMARC, plus mailbox rules that warn on mismatched domains, unexpected reply paths, and messages that pretend to come from trusted contacts.