To overcome email being classified as spam by Gmail, Google recommends a number of steps, which we have implemented (namely SPF, DKIM, and Precedence: bulk).
One additional measure they recommend at https://support.google.com/mail/bin/answer.py?hl=en&answer=81126#authentication reads as follows:
Because Gmail can help users automatically unsubscribe from your email, we strongly recommend the following:
Provide a 'List-Unsubscribe' header which points to an email address where the user can unsubscribe easily from future mailings (Note: This is not a substitute method for unsubscribing).
Documentation for List-Unsubscribe is found at http://www.list-unsubscribe.com/. From this documentation I expect a button to be provided by a supported mail client.
I have tested the 'List-Unsubscribe' header and it does not appear to provide the button. I have tested in both Gmail and OS X Mail. I tested with an http address and with both an email address and an http address. The format of the header is as follows:
List-Unsubscribe: <mailto:unsubscribe-espc-tech-12345N@domain.com>, <http://email@example.com?id=12345N>
No button appears in any test.
How widely is List-Unsubscribe supported?
Should a button be appearing somewhere, or does something else have to be present?
I have seen a comment that even if the button is not present, services like Gmail, Yahoo, Hotmail/Windows Live would give higher regard to email having the header. Thus it might be worthwhile for this aspect alone.
Please note that our standard email footer already contacts instructions and a link to allow unsubscribing from our email.
Finally, is it worth while to implement this header? (That is, any downsides?)