In #2879 you added a header called 'List-Id' to help solve your vacation responder problems.

In #2984 however this change was revoked because it didn't solve the problem with Outlook.

However, I'd like to see the List-Id header brought back, as it gets treated like a first class filter type in Gmail filtering and would greatly help my workflow. I can see that the original problem was around bulk-handling and vacation responders, but this request isn't aimed at that at all.

I would presume the solution would be to bring back the original code in app/models/mailer.rb:

'List-Id' => "<#{Setting.mail_from.to_s.gsub('@', '.')}>"

As show in this revision:

http://www.redmine.org/projects/redmine/repository/revisions/2565/diff/trunk/app/models/mailer.rb