I was asked to build an extremely secure email server.

The specs were:
No email to be sent outside the domain.
No email received from outside the domain.
Highly secured but no use of a VPN allowed. ( Banned in certain locations )

- Only the admin person could see all users.
- Every user can only email people they have been told about by the main admin (who sends them the email address which is a longish random address ( d45f3s5ghe@domain.example.com)
- No mail client access - Originally the intention was to allow use of Outlook, Thunderbird etc but it was quickly recognized that those were less than secure in many cases and it left emails vulnerable on peoples machines.

iRedAdmin allowed me to do this.

Set up highly secured Linux server.
Installed iRedMail
Installed iRedAdmin  (comes with iredmail as well as being able to buy the pro version)

Originally I disallowed imap, pop, smtp access but without imap and smtp Roundcube wont work.
So allowed smtps, imaps

Then to stop external access by clients apps like outlook etc set the only allowable IP for connection as 127.0.0.1 which means any external access to imap, smtp etc is banned. however Roundcube still works as it is local to the machine.

For both sending and receiving white / black lists:
- Set black list of @.  banning all receiving and sending
- Set white lists of only allow to receive from / send to @mysecretdomain.example.com 
- Set 1 ip only allowed to admin users in ../iredAdmin.

This made it so only web access (https only), no external mail clients and no way to send or receive emails from external - even using roundcube.

I then closed firewall ports ( leaving 443 for https and one other for me to ssh)
Set the ssh port to only respond to 1 ipaddress
Setup key access.
Set 2 attempts only to log into email with a bad password.

Setup a bunch of reporting and checking.

Honeypotted all standard areas used to break into a server and set them for instant blacklisting.

Set up no copying, no printing etc on web pages (cant stop screen shots or photos) and let it loose.

I've done similar things previously - but never so easily achieved, maintainable, upgradable and performant as iRedAdmin. I am delighted with how well this roll out for the client has been achieved.

My client does security work - in places through out the world where information going wrong can get people killed or imprisoned. They are not military, govt or similar - but does NGO type work.

I also run three other mail servers for a number of businesses - all iRedMail. it has been rock solid for years. Thanks iRed.