roundcube User unknown in virtual mailbox table

[Samo_slo]

I have two servers an old one that is the production and I want to replace it with a new server. On the new server I installed Roundcube and the ISPConfig plugin and it works great except that I'm getting the error: User unknown in virtual mailbox table.
I get this error only when trying to send mail to mail domains that I have on both server but not the email mailbox on the new server all other work. My servers are Ubuntu 14.04 LTS with postfix and dovecot.

Old server:
domain: 1234mail.si
mailbox: [email protected]

New server
domain: 1234mail.si
no mailbox for that specific address

Did I Fed up in the configuration or  is it imposible? Both server run on different IPs, different FQDN and have different MX records. When I have a mailbox on both servers and I send a mail from a different mail addres on the new server I get the mail only on the new server and not on both?

Hope someone can help?

[JohnDoh]

I get this error only when trying to send mail to mail domains that I have on both server but not the email mailbox on the new server all other work

So the mailbox does not exist on the new server?

When I have a mailbox on both servers and I send a mail from a different mail addres on the new server I get the mail only on the new server and not on both?

Unless you have setup something special for forward a copy of the mail from one server to another then its only going to be delivered to 1 of the servers.