Exim speed up mails send/recieve mechanism

I did this the following way: I have setup a machine in London and China. I have setup mirror GlusterFS for /home were accounts are using Maildir (1 file per email) and it's configured with standard Virtualmin. This way, each email message is replicating from London to China upon delivery or deletion. LDAP is also replicated between sites (the user database). With GeoIP DNS, if I am in China, I download or send to my mail from China server and vice-versa by having the same settings. So this is a working way of doing this for 10.000+ mailboxes. This is not that difficult to setup and works well. With your configuration, you could setup a plain postfix server with virtualmin, which gives you all sort of virus and anti-spam scanning, and then you simply relay all mail to US. The authentication you can make yourself via https query to your CPANEL server, but I never did something like this.