postfix found user invalid

Dear all friend,

I have this scenario as below:

server1.abc.com and I hosted mydomain.com in that server using (exim for sendemail by cpanel)
server2.abc.com and I hosted mydomain1.com in that serve using(postfix for sendemail by virtumin)

my problem is user@mydomain.com or user@mydomain1.com could not send email to each other and I got status as below:

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:
user@mydomain1.com
No Such User Here

and

50-No Such User Here
550 Sender verify failed (in reply to RCPT TO command)

in fact both users already created in both sides.

could you please give me idea about this issue?

Best Regards,