To troubleshoot why your emails are not being delivered to a specific recipient, here are some suggestions:

1. Check that you have typed the correct recipient address.
2. Make sure that your message does not contain any offensive material, which could be blocking its passage.
3. Check with the hosting provider of the recipient, as they might be blocking your emails.
4. Check the time and date of when you sent the email, as it could have been sent outside of working hours.
5. Check the size of your attachment; if it exceeds the mailbox quota limits, your message would not get delivered.
6. Check that the server that you are using is whitelisted.
7. Check that the domain is not blacklisted.
8. Try sending the message again.