Selective SMTP Redirection

Hi,
I am looking for a way to possibly route outgoing mail from different accounts on the same domain to different smtp servers on an account-by-account basis. My reason for wanting to do this is because I am currently setting up a new internal mail server, which I want to test with a couple of test users, while all other remaining users will still have their mail go through the external hosted server.

Here is the scenario:
Domain: mydomain.com
User 1: user1@mydomain.com
Test 1: test1@mydomain.com
External mail server: smtp.hostedmail.com (111.111.111.111)
Internal mail server: smtp.mydomain.com (222.222.222.222)

I want to have outgoing mail from test1@mydomain.com be delivered by smtp.mydomain.com while mail from user1@mydomain.com will still be delivered through smtp.hostedmail.com.

Is this possible with any mail server program, such as postfix, sendmail, or qmail?