Is this using webmail or a third party client like outlook or thunderbird or both?
If you haven't tried webmail try that and see if the issue persists.
If it does, sometimes resetting your IP address can help. To do this turn off your router for at least 30 minutes and then try your emails again.
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).
