Check the following list of items to narrow down the issue:

1. Verify that the email address is accurate: confirm the spelling and format of the address
2. Confirm that the email client is configured to send emails to the correct SMTP domain
3. Check that the email service is not blocked by the receiving provider
4. Verify that anti-spam measures are not blocking the message
5. Check for any log files that might show why the emails are not being delivered