Scaling of imap servers

[Ram]

We need to create a platform for create a large number of cyrus accounts 
that can scale indefinitely
I could start with just 2k  users  but could have 20x the number of 
accounts by next year.
I was thinking of taking a cloud-based machine at amazon or rackspace 
and scale hardware vertically  as required.
But the problem with rackspace or amazon is they do not offer much 
storage and cap the total storage that can be used.


So what is the best way of creating a scalable setup.

If I use cyrus-murder still there is always a challenge of using a 
single mupdate server which cannot handle more than "n"  requests at a 
time.
So I plan to use nginx proxy servers that will just redirect the 
requests and use multiple servers behind it.
For scaling I will have to add more servers behind the nginx , as long 
as the proxy can support it.

Even though this is already working , but I dont see it as a long term 
solution. Backups , HA , DR etc are all not very clean.

I would like to know how do you guys do the same



Thanks
Ram