The problem may result from the Sender Side or Recipient Side.
Sender Side: The mail may not have been sent, or was sent but was rejected for various reasons. This part requires the sender to confirm: whether the mail was sent, whether the correct email address was entered, and whether a rejected email notification was received (common subject lines include: “Undelivered Mail Returned to Sender,” “Delivery Notification: Delivery has Failed.” These rejected email notices give detailed reasons for why the email was rejected.)
The email is recognized as spam or virus-infected by the anti-spam system or mail client.
For the situations described above, if the recipient has an email account hosted by the Center, the user may go to the Mail Reporter to check and resend email, as well as change the blacklist settings.