What version of Dovecot comes bundled with Lion Server?
Dovecot 239 is how it's listed by Apple as open source that was shipped with Lion 10.7 Build 11A511.

You can see lots of nice version and source code information at http://www.opensource.apple.com/release/mac-os-x-107/ in case the 239 doesn't match the real version of the package.

If you want to compare the current with past versions, many versions of OS X are listed at http://www.opensource.apple.com/