Mail flow between Exchange 2003 to 2007

[Axl]

Hi,
We have a single forest and single domain environment (Domain.com). It
has a exchange 2003 server installed in the domain (E2K3.domain.com).
This is the only exchange 2003 server in the environment. The E2K3
server is using SMTP connector with smarthost settings to forward the
mails to the SMTP server which in turn forward the mails to internet.
Recently in order to upgrade exchange 2003 to exchange 2007, we
installed exchange 2007 server (E2K7.domain.com) in the same domain. I
encounter problem when i try to test mail flow between E2K3 and E2K7
servers.
1. Mails are flowing from E2k7 SERVER to users in E2K3 servers but is
not happening other way (E2K3 to E2K7)
i can see the mail in the queue.
to resolve this issue i tried deleting old routing group connector and
created new with bidirectional properties. but still the mail flow
doesn't work. Other thing i did was to delete the SMTP connector in
E2K3 server and created it in E2K7 server. With this i was able to
send mails from E2K3 to external domains, but mails for E2K7 mailboxes
was still sitting in the queue.
Can someone please let me know where i am going wrong.