I'm having exactly the same problem (except for the spam folder), only the last few days.
I do not want to risk trying to change my settings and do not see why I should as it is only btinternet addresses that are sent back after 24.9 hours, apparently by the talktalk postmaster even though my email address is ***@nasuwt.net.
I am not very savvy with how emails are sent but it does seem like a talktalk/btinternet issue.
Can talktalk or bt not just set filtering to accept emails from nasuwt.net?