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Dovecot is great, I've used it before for a few clients and haven't had many issues, but(and don't shoot me for this) I have always been a fan of Microsoft Exchange for the handling of enterprise level email requirements. It has been more stable, scalable, and easier to use from a client side due to AD integration.
Or better yet, you could outsource your email to a hosted exchange provider and get a decent deal on that many email addresses. I have always hated dealing with e-mail due to the amount of client complaints, most of which are client side issues. Thats just my personal thought, some people love SMTP response codes and walking folks through outlook setup.
Not that my mail server has the amount of users your asking about. But I love qmail with Dovecot.
You didn't ask about the MTA, but qmail has the ability to block a large number of spam sources through RBL, Greylist, greet delay, blocking by IP, and your own Whitelist/IP. I have used these combinations for around 5 years with no problems.
I worked for a web-host provider and we used qmail/dovecot for our enterprise level email services as the MTA and it had quite a few problems when trying to handle the amount of emails generated. I love qmail for the individual as it has a lot of flexibility but I personally suggest postfix for enterprise level stability and scalability. Again, just an opinion I'm sure you wouldn't be in terribly bad shape if you went with qmail or even exim.
We use postfix as MTA too. Right now we aren't thinking about changing because postfix is quite stable in our infrastructure. What we really need is a new IMAP server that is stable, modular, scalable and has a good performance in a scenario like ours.
From this thread I see it is mandated to run on linux, but does that mean it has to be free and open source too? The old Sun/iPlanet software is now owned by Oracle, but it is "extremely" scalable, modular, reliable, flexible, etc. 10,000 simultaneous connections are small potatoes in the solaris environments I have worked in, but with LDAP and clustered mailbox servers on the back end storage layer, and load balanced SMTP and POP/IMAP proxies on the front end access layer, you could probably offer your customers 99.99% uptime (assuming power, cooling, and other dependent components are also redundant to the same level) even in linux. Considering your entire IMAP population is only 10K, 1,000 simultaneous connections would probably be peak load. You may want to consider your traffic and architecture, as well as population and load to get the most effective solution for your environment's needs.