Mailpile
Mailpile provides a nice looking webmail interface suitable for use on desktop or mobile clients. It has good support for email encryption and makes that quite an simple process. At present it's usable but still has a few bugs and limitations. If you need a fully functional email client with comprehensive encryption support then either use Mutt or Thunderbird/Icedove.
An advantage of this type of webmail is that it keeps your GPG keys off of any mobile devices so that if you lose your phone, or it gets stolen, then your email might still not be compromised.
One down side is that this appears to be a single user system, so if you have multiple users on your Freedombone server only the administrator will actually be able to use mailpile.
Setup
If you're viewing your mail domain site on a mobile device via OrFox then make sure you allow the domain in the NoScript settings.
Enter a password and store it within a password manager.
Click on the Privacy and Security button.
Scroll down and select Save Settings. Don't click on the Tor button.
Click Add account.
Enter your name, email address and password.
Uncheck Detect Settings and click Next.
Under Sending Mail select local or if you need to proxy outgoing email through your ISP's server select SMTP/TLS and enter the details, then click Next.
Under Receiving files select IMAP/TLS, your main domain name, port 993, your username and password, then click Next.
Under Security and Privacy either select your existing encryption key or if you only get the option to create a new one then do so, then click Add or Save.
The process of importing your email should then occur, and can take some time.