Notifying a user that an email message sent to the user was not received by the user. Users can register to receive notifications. When a sender sends an email message to a recipient, a separate notification also may be sent to a notification service that populates and controls access to an email notification list. Each notification in the list may comprise a hash of the content of the email message that it represents. A recipient module may be configured to check the notification list periodically and/or in response to a recipient's instructions, and compare notifications to the email messages received for the recipient to determine whether any email messages intended for the recipient were not received. If it is determined from the notifications that an email message was not received, a notice may be sent to the sender, which my include a request that the sender re-send the email message.