[Dovecot] Why dovecot is not multi-threaded?

Hi,

I'm looking for the most efficient OpenSource IMAP server and I am told that Dovecot Server is a good solution.

Anyway, I have inspected the source code and I think Dovecot is not a multi-threaded server, but a forked server. I am surprised about this issue, because threads perfomance should be higher than creating a new process for each connection....I suppose.

So, Could anybody tell me why Dovecot was not designed with multi-threaded support?. Perhaps, threads are not a good solution for Dovecot but I am very interested in which are the reasons for this.

By the other hand, ¿do anybody know any multi-threaded OpenSource IMAP server?. I would like test performance of Dovecot server versus a multi-threaded one.

Very thanks.