Need help: SMTP 550 ERROR failed to add recipient

==== Required information ====
- iRedMail version: 0.8.3
- Store mail accounts in which backend (LDAP/MySQL/PGSQL): mysql
- Linux/BSD distribution name and version: debian 6
- Related log if you're reporting an issue:
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Hello, I'm a newbie and I am having a trouble with sending email between 2 email server:
I create the email account for both of them, sending in local is ok, but when I try to send between 2 email server "An error occurred".
"SMTP error( 500 )": Failed to add recipient name@example.com (5.1.1 <name@example.com>: recipient address rejected: user unknown in virtual mailbox table)."
So, what direct for me to solve this issue?