is a term used in the e-mail marketing industry. A piggyback e-mail is comprised of a third-party advertisement embedded in the body of a e-mail sent to an opt-in subscription e-mail list
that is comprised of demographics of target consumers the third-party want to advertise to. In this instance, the e-mail containing the advertisement comes from the list owner, not directly from the advertiser.
See also solus e-mail.