SMTP error from remote mail server after end of data

Dear all, please hlep me, i just setup new WHM&Cpanel for testing, but i try send email test to gmail, it's reply error with this message in mail log : 1gof6H-0002m5-Tv ** ki*****@gmail.com R=dkim_lookuphost T=dkim_remote_smtp H=gmail-smtp-in.l.google.com [172.217.194.26]: SMTP error from remote mail server after end of data: 550 Requested action not taken: mailbox unavailable I user my local mail server in private IP and make share ip with public IP and Nat from Firebox watchgaurd, so i'm not experience with this. i'm not make sure i wrong configure with firebox or what.