Mailpile
Mailpile provides a nice looking webmail interface suitable for use on desktop or mobile clients. It has pretty good support for email encryption and makes that quite an easy process. At present it's usable but still has some 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, 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 click Next.
Click on Inbox and then select Browse for mailboxes. If you have a lot of mail then this can take quite a long time. Meanwhile you can giggle at the messages under Working….
Importing existing GPG keys
Unfortunately this is broken in the current Mailpile version, so you can't import keys. But when it works this is probably how it will work:
ssh into the Freedombone with:
ssh username@maindomainname -p 2222
Select My encryption keys
Select Show full PGP/GPG key. Select the text of the full key and copy it, then paste it into a file on your local system.
In a browser navigate to yourmailpiledomain/contacts
Select import encryption keys followed by upload key
Click select encryption key then choose the file you previously saved.