BoltMail
How does the unsubscribe function work?
The application includes the ability to generate a "modify details" form link for each newsletter, so your contacts can simply click this and then change their name, email and also choose which lists (single or multiple) they would like to unsubscribe from.

The application also includes one-click automated unsubscribes by adding a placeholder to the bottom of your email. The application can do this for you automatically if enabled from the settings page.