You can set this value to “true” in your Bulk and Transactional API calls and it will cause a “List-Unsubscribe” header to be added to your email message(s). This header has special meaning to several ISPs and Email Readers (most notably GMail). If present, it provides the information needed to unsubscribe an email address from receiving future mailings.
Example Unsubscribe Header
List-Unsubscribe: <http:>, <mailto:firstname.lastname@example.org>
Some ISPs will send an email message to the “mailto” address in the header. Others will invoke the url in the “http” section of the header. In either event, you will receive an “unsubscribe” activity in your daily or real-time data feed.
GMail’s Use of the Unsubscribe Header