A List-Unsubscribe header is a header added to an email which instructs the recipients email client how the email will process an unsubscribe. Some email clients will use this to display an unsubscribe button prominently on the email, outside of the email content.
Some email servers (e.g. Yahoo) require this for any bulk mail senders, and without this there is a high chance, your emails will be blocked.
OpenCRM will automatically add a List-Unsubscribe header if you add a remove me or manage preferences link from the click through wizard, using the same link for the header.
If you do not wish to insert an OpenCRM unsubscribe link, or you want to override the List-Unsubscribe link, you can do this by adding the following html comment anywhere in the source of the email:
<!-- UNSUBSCRIBE-LINK="https://yourwebsite/unsubscribe" -->