Citadel works on a Unix-like Server and windows clients can be connected.
Regarding Squirrel mail contact
deeprootlinux for paid support. else free support is available online
[
DeepRoot Linux] Location: Bangalore, India
We're home to a pioneering effort to deliver quick to deploy and easy to manage gnu/Linux based server software to you. We've spent the last five years developing great gnu/Linux server software, providing support and assistance to more than 100 organisations in deploying, using and managing gnu/Linux and Free Software and have a lot of fun doing all this.