Email problem

[shaking]

When I email a certain address then send it, I get a message saying ''mail sent but could not copy to designated sent folder''
All other emails I send are in the sent folder apart from this one address which I email every day.
Also all the folders are showing (o) when in fact there are loads of mail in them.
I am not getting any joy from my provider talktalk whatsoever.
Can anyone help please, tell me what to do to the computer to rectify the problem.