I would recommend xmail as a production intranet email server on windows because it is very easy to configure and administer while being quite feature rich. Mercury is a bit tougher to administer, but has more features.
Mercury mail transport is what i ran for quite a while, before i managed to get a dedicated box to run linux on. I'd also note it plays well with quite a few AV solutions as well. Its also been around for ages, and has been fairly actively maintained the whole time as well."
