Mail Logger, Drupal's most have module for all websites
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