Many times while sending out mass e-mails, a few of the recipients will wish to stop receiving further e-mails. By default, e-mails sent with the Mass Email Wizard have a link at the bottom, which the recipient can click to unsubscribe from further mailings. The link will direct them to another web page. The message on the web page will be what is under your Settings / Account Settings / Email Information / Unsubscribe Message box.
You can see a list of the unsubscribed people by going to Settings > Unsubscribe Manager. The unsubscribed e-mails will be listed, along with the day the email was unsubscribed.
If someone was unsubscribed accidentally, or someone wishes to resubscribe to the emailing list, you can resubscribe them by going to the Unsubscribe Manager and clicking in the subscribe column for that e-mail address. You will be prompted to send a resubscription e-mail with a special link, so do not change the existing text in blue. Once the recipient clicks on the link, they'll be linked to another web page. The message on the web page will be what is under your Settings > Account Settings / Email Information / Re-Subscribe Message box.