DISCLAIMER – I'm fishing for ideas, so I can see that some people will start downvoting and flagging this as unsuitable for SF, but hey, if you know of a better place to ask this question, there is a comment box down there, don't just hide behind your clicks.
I have this tiny production server that I'm sharing with colleagues and close clients. I've been asked to provide an e-mail service. I guess I never thought about this, but I was horrified to discover that the various mailbox formats store messages in clear text, and close to nobody seem to care very much. So I figured, those fancy providers must be doing something behind the scenes. I'm not necessarily thinking of ProtonMail, but I guess Google does store messages with some form of encryption – and yes, I realise these services might have developed their own software, but hey. So I started reading up, and the closest I got was a relatively complex Exim4 setup involving a GPG
transport_filter with per-user key pairs.
The truth is that the messages that will likely go around the server will not be very confidential, and my colleagues and clients are savvy enough to use GPG themselves if need be. My main concern, rather, is that these messages are available in clear text for anybody – I can see this happening as soon as I won't be the only one managing the server, the occasional
cat /var/mail/username (or whatever) will happen. Worst of all, the messages will be backed up in clear text – true, I can encrypt the backups.
As I said, I don't want to provide a protonmail-like service, I'm just looking for something that provides some form of smokescreen against careless admins. In particular, some form of clear text will be necessary during transit, for stuff like anti-virus and anti-spam. However, if you have compelling and uncomplicated solutions for something more secure, I'm open to all suggestions.