Unfortunately there is no way to know that the email has been correctly delivered right after sending it. Therefore when someone send a message using Drupal's Contact form or any other modules, he may receive a "Successful message" but the site owner may never receive that message due to the wrong email address , network failure , mail server problem or blocking by spam protector.
This might cause some misunderstanding , loosing a great job opportunity or etc.

There is a very useful module called Mail Logger which addresses this issue by storing a copy of all outgoing emails in database. So administrators can check admin/reports/mail-logger for example once a day for finding and reading the missed emails.

I strongly recommend this module , Kudos to litwol for sharing it