Anyone running on Ubuntu (or Mac)?

[Mark Dorset <mdorset@trinity.unimelb.edu.au>]

Hi,

We're mainly a Debian Linux server environment, though have been  
running Zimbra with a small number of mailboxes for a couple of years  
on a Mac OS X Server. We're considering an expansion from our roughly  
40 mailboxes to around 300 and thinking that a linux based server  
might be better. Given Ubuntu shares many similarities to Debian it's  
the obvious choice, but I thought I'd ask if anyone here has any  
reason to suggest using caution over using RHEL, or any other UNIX  
variant? (If anyone else is using Mac OS X Server on a medium sized  
deployment and wants to convince me that's the best choice too, I'm  
all ears).

cheers,
Mark