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.
From the 29th of March our TalkTalk Community will move to peer-to-peer support and the TalkTalk team will no longer be here to support with queries. Don’t worry we still have plenty of ways to reach out if you need to speak to TalkTalk directly. For more details check out the link here:
How to contact TalkTalk Broadband - Help & Support.
