I know we have several customers running larger systems than 10k users, and using CE to do it.
Our sales engineers are pretty good at helping size the system, so you get good performance.
Messaging Server itself is fully able to handle mail stores with near a million users for standard e-mail. I have some users with up to 2 million users per store.
I have over 15,000 users in my installation. The key question here though is how many users will be connected at any given time. How many imap, pop and Communication Express users. It is not necessarily the total number of users that will cause you problems, but how many users are simultaniously on. Also how many messages do they send and the total volume of email coming in.