server email, 511 sorry, no mailbox here by that name .. but the mail exists!

Hi, as the title suggests when I try to send an email from my server to the domain: @ [login to view URL] I am sent back a failed delivery email with the following error: Delivery to the following recipients failed permanently: * PZIC84100D@[login to view URL] Reason: 2idYlUlgkuxbd2ifSluhDP We're sorry, your message was not delivered. The receiving server replied: 511 sorry, no mailbox here by that name (# 5.1.1 - chkuser) but the email exists! in fact if I try to send it from gmail it arrives without problems! however, if sending from this server to other addresses works without problems