[exim] Exim Relay Emails to optimize email send / recieve me…

Hi,
We are using Exim MTA on dedicated server hosted on US data center with
CENTOS 5.
Our users are in two regions in US and China. Problem is that we have
connectivity issues from china to US DC and because of these china users
having difficulties for sending emails along with attachments most of the
time because of slow connectivity. Because china users have to connect to
US DC SMTP server and because of slow connectivity issues users suffers
alot.


Currently main mail server which hosts all users mailboxes. and i don't
want to separate domain name between two datacenters. I still think
separate SMTP server on both datacenters can solve email sending issues but
how to accomplish this without changing any user email domain e.g.
abc@??? or any other idea ?

Actually when users from china tries to send email along with attachments
it takes quite some minutes infacts sometime quite long because end user
need to connect from china to US DC primary MTA and because of connectivity
and bandwidth issue users face problems. However, as i said earlier, we
also have machine in China DC and connectivity b/w china & US DC is much
better.
I want solution something like when user sends email from china by using
our domain @example.com it should connect China DC (we will install exim on
china DC machine if require) which will responsible to send email to US DC.
Also we don't want to switch china user mailboxes to China we need to keep
all user mailboxes to primary MTA.
Through this we will foresee email sending will speedup atleast.

Also I need to know how to do it to receive emails ?
Please let me how to accomplish this or any other solution?