You say your emails to Gmail recipients aren't delivered? How do you know? Do your contacts tell you? If messages from TalkTalk Mail are not delivered then you should see a bounceback delivery fail notification.
I'd suggest your messages are delivered. TalkTalk Mail is fully authenticated so no reason not to deliver. Gmail is simply dumping mail into spam folders as a policy. I suggest you ask your contacts to look in their spam folder. Then select your message and tap the Report as not spam button. This will tell Gmail not to dump your messages into spam but to deliver your future messages direct to Inboxes where they should have been placed anyway.
