If the recipient is using a home internet account (rather than being at work) ask him to log into his email account via his ISP's webmail service, instead of using an email client (such as Outlook Express or similar). He will then probably find a folder labelled 'Junk', 'Spam', 'Bulk' or something similar. (That folder won't be present on his email client). Your missing emails will probably be in that folder, together with an option to mark them as 'not spam'.