Ah good old squirrelmail :P A recommendation: https://www.open-xchange.com/ seriously gorgeous webmail system, document collab/storage/editing built in - totally replaces the need for a desktop client
Ich habe zwar keine Ahnung wie die Software mit dem Pi zurechtkommt und sie auch noch nie selbst aufgesetzt, aber mailbox.org und Strato verwenden https://www.open-xchange.com/portfolio/ox-app-suite/, die ziemlich stabil läuft und einen Haufen Features hat.
Zimbra has decent uptake in companies. You could also have a look at open-exchange's offerings, though their SAAS approach is quite far-reaching. https://www.open-xchange.com/portfolio/ox-app-suite/
Yep, we switched to using Open-Xchange. If you want all their features you have to pay for it but they do offer a community edition; https://www.open-xchange.com/. At first glance it looks like a full stack but it's just a webmail client on steroids.
I'm also with /u/jerutley on this one. While I think the paid Google Apps is worth it's money, Zoho offers a free alternative for your own domain name. Also almost all web providers offer email services with the webspace. Some of them are very reasonably priced and still offer a great email experience. Some even go as far as offering a whole office suite. The OX App Suite is a white label product targeting hosting and internet service providers. GMail isn't the only game in town.
OTOH if you really want to do it, Kolab has a great groupware suite with excellent CentOS support for their FOSS offering. Setting up an email server can be a great learning experience.