ok, as it states, just looking for a mailserver guy or 2 that would suggest a stable distro. 35+ servers are all linux based (various distro's) but one lingering MS server left used as a mailserver (mDaemon).

I finally got the ok to move it off windows, so I have played with postfix a little, but want to get an OS on, will take a domain we have not in use and get that running. I looked at Zimbra, but really I need a basic mailserver, just need the following;

1. webmail (roundcube looks cool, but in beta.)
2. multiple domains & user alias's
3. imap/pop
4. imap filters (not sure on this one, users have a lot of IMAP filters setup and would love to just get them setup vs. telling them to use a client app to do this)

That's really it, so it's pretty basic aside from the rules, but I am hoping all is pretty simple and available using postfix (or another suggested linux mailserver). So with that, any disto's that are and are NOT recommended is appreciated.

Any of the more popular distros will work. CentOS, Suse, Ubuntu, Debian. It really doesn't matter. Each person has his favorite but they all use the same kernel.

Choose one with which you are familiar.

For a server your primary concerns will be security and stability. In many ways the two will go hand in hand. Personally, I would look at the following for servers, in no particular order or preference: debian, slackware, and RHEL. I have used the Ubuntu server edition I recommend both debian and slackware for their reputation of stability which includes both operational and support. I mention Red Hat because of its corporate leanings and if you are using this in a business environment, the support may be of interest. I personally use Ubuntu server edition, which I didn't recommend. While I have been very happy with its performance, I find the 6 month release schedule with support dropping off to be a problem with a long term server implementation. The one time that I did update my distribution, I found that certain process stopped working and it required "fixing." If you do go with Ubuntu, which will make a fine server, be sure to choose a LTS - long term support edition.

In your particular case, I think just about any distribution would do and you should consider familiarity as well as reputation as this will ease the maintenance process.

I would also suggest not running any more processes than necessary. In my opinion, this includes a graphical interface which I don't use on a server. Slackware has an advantage here in that the default is to not run X, but you have the option should you so wish. For your application, a GUI should not be necessary, but having web base PHP interfaces may be a bonus.

Hope that helps.