Email bounced back

Whenever an email is sent to "name.lastname@mailbox.org" from another @mailbox.org email address, the email is bounced back with the following message: "Delivery has failed to these recipients or groups:
First.Last (name.lastname@mailbox.org)
 Received message by the sender: "The email address you entered couldn't be found. Please check the recipient's email address and try to resend the message. If the problem continues, please contact your email admin." However, the recipient still receives the email. When an email is sent externally, such as from a Gmail account, the email is received by the recipient and no bounce back to the sender. 

Is there any advice regarding this?

Kr,