looking for a mailserver recommendation:

If you're going to use ispconfig as a hosting panel then you'll have to use one of the mail servers which it supports: Sendmail or Postfix. Of the two, my preference is for Postfix.

I started out running my own ingoing and outgoing postfix server. I switched to postfix for outgoing and Google Apps for incoming. I will not be going back.

And actually, I could never get yahoo to respect my email, even after installing spfs, domainkeys AND dkim, rdns etc etc. So for really important emails (invites etc), I SMTP to gmail and send using their servers.

Anyway, running an ingoing mailserver is a time-consuming and thankless job.

I do the same as deadwalrus, I use postfix for system notifications and google apps for everything else.

Google Apps is a good solution; I use it for some domains. But I also have clients who simply do not want their mail there. So I provide in-house mail with Postfix and Dovecot. And yes, they thank me, so I disagree about it being a thankless task :)

Try Citadel. On Ubuntu sudo apt-get install citadel.

http://www.citadel.org/doku.php