It's worth noting on top of what others have said, Gmail (and assume most others too) will only display the unsubscribe button from whitelisted mail servers, as far as I'm aware.
The reasoning being that it could be used by spammers to confirm an email address is real/valid after a user attempts to unsubscribe at which point they could sign them up to more spam. So don't expect your own emails to show the button by just adding the List-Unsubscribe header, unless you're using something like Amazon SES.
The reasoning being that it could be used by spammers to confirm an email address is real/valid after a user attempts to unsubscribe at which point they could sign them up to more spam. So don't expect your own emails to show the button by just adding the List-Unsubscribe header, unless you're using something like Amazon SES.