Ex-speed.,
I do use bt internet for some of my mail, and I use Outlook 2007.
Every now and then, Outlook plays up with BT, so I have to delete my i.d. and start again. Now I know how to do it, it is easy, but the first time required consultation with all the Windoze advice.
From this, I can only suspect that your Outlook is having interaction problems with btinternet. However,as that address is at the recipient's end, I have no idea whether it is bt or your outlook that causes the problem.