It's a known issue and Microsoft and Google are working with TalkTalk on a solution as it's primarily affecting Gmail and Hotmail domains.
It's intermittent and is proving hard to pin down.
Best advice at the moment is to add sender's to your email address book and your recipients do the same.
Please remember to mark Solved Posts with Best Answer. Doing so helps other customers and saves TalkTalk's Support Team time by only looking at unsolved topics. Thanks, Steve (a fellow customer).
