The only cause that I can think of is that your friend's email service has a spam filter in place which is blocking your mail.
Try to send your friend an email from the account you use with Windows Mail, then ask him to access his mail account via TalkTalk's web-based system (instead of via his email client, such as Outlook Express or Windows Mail). The link he needs is here:
http://www.talktalk.co.uk/mail/?check_cookie=1
There's probably a folder labelled 'Spam', 'Bulk' or something similar. He might well find your email in there. If so, he needs to look for an option to mark the mail as 'This is not spam' (or something similar).
The foregoing is written from experience. I've had plenty of problems with emails from trusted sources going missing,only to find that they've been blocked by spam filters.
Chris