There are two email servers. It is necessary that some boxes were stored on the first server, others on the second . They should be allocated in one domain @qwerty.com.
How can i make it? Configuration: postfix,cyrus,sasl,debian
For SMTP part you can use lookup tables in Postfix. Depending on your choice and number of user accounts you can use local files, MySQL, or OpenLDAP for storing the lookup info. I have done this with OpenLDAP and it works great, even though the initial setup can be a bit painful.
For the POP/IMAP part Perdition is a nice choice. It can also retrieve the user mailbox location from local files, MySQL or OpenLDAP.
Another way to distribute the mailboxes would be to use Cyrus Murder, but I'm not sure how stable it really is. Some say it is, some say it is not.
This document describes a high capacity email system. You could check if it matches your needs or gives you some hints