One cannot really say that Google is not working hard to make Gmail a better service. They started to introduce many features in the online email client lately that would not only be offered as an experimental widget or function in Gmail labs but also some that were introduced to all Gmail users at once. We mentioned a few days ago that Gmail started displaying images for trusted contacts right away which in itself was a nice addition to the email service.
Another new service was introduced only a few days later. This new service allows Gmail users to automatically unsubscribe from email newsletters. All that needs to be done is to either report the email as spam which will bring up the unsubscription option or to click on the Show Details link in the top right corner and to click on the appearing Unsubscribe from this sender link (if it does not appear it means the newsletter cannot be unsubscribed from automatically).

This sounds – in theory – like a good new feature but there are a few limitations currently in place.
The unsubscribing option is only available for some legit newsletter providers. This is understandable as it would not make sense to unsubscribe a user from a spam newsletter as it is likely that the user did not signup for that newsletter in first place and that the cancellation request would result in more spam than in less because of the verification of the email address.
Not all legit newsletter providers are supported as well at this point. The Gmail blog gives some information on how to prepare a newsletter for the automatic unsubscription. It is definitely a welcome feature that needs broader support to become effective for the majority of users.

Send a Fax to unsubscribe from paypals newsletter
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Gmail Starts Displaying Images Automatically For Contacts
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Gmail Translations: Who Needs It?
There will be broader adoption by senders if/when gmail decouples the “unsubscribe and report spam” option. Granted, the “unsubscribe only” option is buried under the View Details menu, most users won’t likely know it’s there. Most legit US senders already provide an unsubscribe link in their emails as is required by CAN-SPAM. It’s doubtful many senders will want to support List-Unsubscribe headers if they’re going to get dinged for it (spam-wise) in the end.