Email problem 550 - 2 websites at one server, but mail at different

Hello, we have a problem with sending mails only with clients which hase website on the same server (on server but 2 different cpanel). One mail is on this server, but another is on external server. [QUOTE] This message was created automatically by mail delivery software. A message that you sent could not be delivered to one or more of its recipients. This is a permanent error. The following address(es) failed: xxxx@xxx.pl LMTP error after RCPT TO:: 550 5.1.1 User doesn't exist: xxxx@xxx.pl
For example we have like that: One dedicaded server - 2 different cpanel account for2 domains. domain1.pl domain2.pl DNS from domain1.pl going on our server, and we have here website and mails. another domain DNS ar going on different server, (because mails are on different server) but record A (website) going on our server. We can't send emails to peoples from domain2.pl because we have error. What we have to do? name of mail adress id correct.