You mention that you use a hardened Linux OS, but you don't discuss any of the other opensource software that you use. While you technically only have to distribute the sources to the people that receive your hardware, it would be nice to know these kinds of things before purchasing. This is a niche market you're marketing to that has a history of researching these kinds of things.
Are you using a custom distro or one of the off the shelf ones? What webserver (presumably there is one) is running on this? MTA? What're you using for backups? Is this just a ownCloud running under the hood with a tunnel to AWS and small proprietary layer on top?
Along different lines, how to do the backups work? Are they just stored on the device (what happens when its drive dies)? If not where are they stored and how much manual intervention is required. If your company goes under can I use any opensource software to decrypt the backups and access my emails?
Edit: With the list of sources in hand I can answer a lot of the questions I posited. So for others interested:
Distro: Still unknown, but see additional interesting tidbits.
Webserver: Neither nginx or apache are present. If I had to guess its using golang's net code to run any webserver / API that is present
MTA: postfix, dovecot, opendkim, opendmarc, spamassassin (might be more components I missed)
ownCloud: Doesn't appear so
backup software: Duplicity
backup location / process: Unknown
Interesting other tidbits:
OpenVPN is present, and so is ipsec-tools/strongswan. So they might be using either IPSec, OpenVPN or a combination of both for the tunnels. They could just be distribution defaults.
It's running docker, I'm guessing they're using some form of container image sync update system rather than a proper package manager.
It contains a fully developer toolchain, so presumably this is a full distribution rather than a minimized / custom one.
Based on the versions it appears the ARM processor is a QorIQ variant which is an ARM evolution of the PowerPC architecture.
Contains the libatomic libraries, so perhaps this is running on Fedora Atomic as an underlying OS? Would explain the presence of docker and give an idea how they're providing updates.
Appears to have a wifi chip (I couldn't find any information about connectivity on their site). Specifically it seems to be using a Qualcomm ath10k chip.