An email was sent out, but the recipient never received it. They also checked their junk folders. Why did this happen?

If we see the “Received Email” metric on the recipient’s profile, it means that the email was received and queued for delivery by the recipient’s server. This does not necessarily mean that the email reached the recipient’s inbox. If the recipient did not receive the message, or if there was a delay in receiving the email while it is showing “Received Email” on the profile, one possible reason could be a filtering program they make use of on their end. If their filtering program is preventing the email from reaching the inbox, they should try allow-listing their dedicated sending domain. Either way, any more information regarding the status of that email can only be provided by the recipient's ISP.