Email spoofing is the creation of email messages with a forged sender address.
Email spoofing is the fabrication of an email header in the hopes of duping the recipient into thinking the email originated from someone or somewhere other than the intended source. Because core email protocols do not have a built-in method of authentication, it is commonplace for spam and phishing emails to use said spoofing to trick the recipient into trusting the origin of the message.
Email spoofing is when the sender of an email, typically spam, forges (spoofs) the email header “From” address so the email being sent appears to have been sent from a legitimate email address that is not the spammers own address.
To trick spam filters into allowing the email through by using a reputable email address. This would be one way your friends and family would see spam emails from you in their Inbox, rather than their spam folder.
To prevent the bounce back emails from being received in the spammer's own inbox. Spammers may send their spam out to thousands of email addresses, and inevitably a lot of those emails are going to bounce. Since spammers don't want to receive hundreds of bounce back messages, this prevents that from happening.
Email spoofing is more common with email accounts that are not actively used. If the account is used on a daily basis, there's a higher chance that your account might have been compromised by malware or a virus.