Is this just Gmail recipients? We know that Gmail have a new anti-spam policy that dumps mail into Spam folders without even checking if the mail is genuine or not.
There's nothing that you can do to stop Gmail doing this. TalkTalk say they've done all they can. All mail sent is fully authenticated.
When you do make contact with your recipients and they find your mail message(s) in the Spam folder just kindly ask them to select the message and then select the Report as Not spam button that will then tell Gmail to place all your future messages to that recipient in the Inbox.
