I may be wrong but I understood .ac.uk to be a university (or other large institution) suffix. Universities have amazing spam filters and I had the same problem trying to contact my brother who worked at a uni. It was only resolved when he emailed me first and then I used that to email him.
sedum is right, it's a university account. It may not be anything to do with talktalk but it might be - I have no problem emailing my friends and business contacts on ac.uk addresses but I'm not on talktalk.