I too am having the same problem except it is only certain e-mail addresses. I am also with talktalk and if I use their web based e-mail there is no problem. It is only when I use outlook that certain e-mails are not received.
Like above it is recorded in the sent folder and no error meassages but the recipient does not get them. It was only blueyonder that there was a problem with but now in the last 2 weeks there are more.
Can somebody please find a solution to this as using outlook is becoming useless !!