I'm in the UK and have a gmail account, but that is held in the US, I also have a hotmail account, but that has a hotmail.co.uk suffix rather than the hotmail.com one, so I assume that's using UK servers.
I'm not at all sure that it's possible to what you are trying to do. The problem I see if this:
Server and users in America
Server and users in UK
Internet user in Australia sends an email to one of your UK users, how does the Australian ISP know to route the email to the UK when the DNS suffix is the same as the US one?
Or, the same user in Australia sends an email to all of your users, which server does it go to?
Hopefully someone else will have a better handle on this, but I cant see a way to make it work at all, let along work reliably. I'd have to say the answer to any problems you may be having would be better addressed with an increase in bandwidth rather than installing servers in a different country.