If you're not received a 'bounced mail' message, that strongly suggests that (for some unknown reason) the email service used by the intended recipient is treating it as spam. (It happens a lot. I once had to try lots of different ways of sending email to an American friend before finding one that his email service didn't simply delete. Then I found that the email service I was using wouldn't accept incoming mail from him!).
Get the intended recipient to log into his/her email account using web-based access. (i.e. NOT using an email client, such as Outlook Express). They'll probably find a folder labelled 'spam', 'junk' or similar that contains recent emails that you've tried to send to them. There should then be an option to mark them as 'not spam', so that the email filters will no longer hold back your mail.