See forum topic http://forum.civicrm.org/index.php/topic,979.0.html
I suggest to make an admin 'unsubscribe', 'opt-out': set up choice:
1: mail method (current) (with smtp, postfix or imap2soap)
2: webpage method (with or without smtp, postfix or imap2soap)
3: no 'unsubscribe', 'opt-out' feature for users (no check of sender in newsletter script to)
May be option 3 is debatable. I like the 'unsubscribe', 'opt-out' obligation. I understand it is law in some countries. I think there are other countries were it is a free option. My opinion, quality based, is to force CiviCRM admins to use the 'unsubscribe', 'opt-out' option.
1 When a user clicks on 'unsubscribe' or 'opt out' there is a email check (minimal) first
2a When smtp, postfix or imap2soap is installed the check is automaticle
2b When smtp, postfix or imap2soap is not installed the user goes to a web page where he has to fill in a form (profile setup) with minimal his email adres
3 After verification he/she goes to a a texst-customizable webpage with the text: "Are you sure you want to unsubscribe? Y/N/Cancel (button). In case of opt out: "Are you sure you want to unsubscribe? Y/N/Cancel (button). This page is not reachable by anonymous users.
4 When clicking Y go to a text-customizable "sorrie for leaving us, thank you .." page with a finish button: closing browser window.
You can also make a pop up thank you window, disapearing after a predevined time (admin setup).
5 Send a (admin/user defined optional) email with a conformation of the opt-out/unsubscribe
6 When clicking N close the browser window. May be pop up with "glad you stay with us"
7 When clicking cancel: break the proces and go to a pre defined webpage for further surfing
The 'opt-in', and 'subscribe', feature is a part of Profile configuring. And therefore not mentioned in the workflow and setup.
Re-opt-in and re-subscribe could be a feature to think about when an email with a conformation of the opt-out/unsubscribe is send. This email must have the re-opt-in or resubscribe webpage included. The Re-opt-in and re-subscribe is also a part of Profile configuring and therefore don't need a workflow proces.