未經你的同意而寄送到電子信箱裡的信件就叫做垃圾信 (也稱為廣告信、大宗郵件)，英文稱為 SPAM，而放送 SPAM 的不肖人士則稱為 SPAMMER。
垃圾信之所以會有「SPAM」的名稱，是從英國的影集而來。劇中一群人大聲高唱 "spam, spam, spam..." 企圖壓倒其他聲音，跟垃圾信用大量寄發的方式強行寄到收信者的信箱，用意是非常類似的。
從事行銷廣告的人士則說「SPAM」這個字是「同步刊登廣告訊息」的縮寫：Simultaneously Posted Advertising Message。
根據2003年2月9日紐約時報的報導，1978年 Digital Equipment Corporation 公司的業務員發出的產品介紹是世界上第一封的電子垃圾信。
Detail: 550 SC-001 (COL0-MC1-F28) Unfortunately, messages from 114.34.X.X weren't sent. Please contact your Internet service provider since part of their network is on our block list. You can also refer your provider to http://mail.live.com/mail/troubleshooting.aspx#errors.
Client host [114.34.X.X] blocked using FBLW15; To request removal from this list please forward this message to email@example.com
Thank you for contacting Microsoft Online Services Technical Support. This email is in reference to ticket number 45465466 which was opened in regards to your delisting request for 114.34.X.X
The IP address you submitted has been reviewed and removed from our block lists. Please note that there may be a 1-2 hour delay before this change propagates through our entire system.
We apologize for any inconvenience this may have caused you. As long as our spam filtering systems do not mark a majority of email from the IP address as spam-like, your messages will be allowed to flow as normal through our network. However, should we detect an increase in spam-like activity, the IP address may be re-added to our block list.
Should you have any further questions or concerns, please feel free to respond to this email.
Thank you again for contacting Microsoft Online Services technical support and giving us the opportunity to serve you.