This topic has been beaten to death on HN over the last year (other people can provide links to discussions, with Moxie participating). I think something worth keeping in mind is that almost everyone who works in secure messaging agrees on one thing: that electronic mail is not the future of secure communication. There's no fundamental reason why that should be the case. The store-and-forward model used by SMTP could…
One important benefit of email is that you're not reliant on any single provider for your communication. Slack and Signal and systems like them are provided by single companies, and the continued function of those systems depends on the companies. Your long-term happiness with those systems could be influenced by how those companies interact with foreign or domestic governments' requests for surveillance, for censorship, etc. What will happen if one of these companies is ordered by a government to, like Lavabit, backdoor their system, or defeat the encryption on a device like Apple? Perhaps it will happen through secret court order and we'll never know, or perhaps the company will decide to shut down like Lavabit - as a user both are bad options.
With email, you can host your own email server, and take the fate of your group's secure communication into your own hands. For anyone to get your data they'll need to come after your devices directly; they can't just send a court order somewhere to obtain information about your communication - not even metadata. Although systems like Signal may not have the preexisting ability to eavesdrop on the substance of your communication centrally, they may be able to eavesdrop on your metadata and contact list, and it's possible that they or any vendor involved in app distribution such as app stores, OS vendors, telecom providers, device manufacturers, could be ordered to collect your data or metadata, or even build and deploy a backdoor.
As far as security, I think email is adequately secure for private group communication for most purposes. Many of us do rely on email for just that purpose within our organizations (e.g. companies). If your organization has a central mail server that authenticates its members, then you can get to a pretty good state fairly easily. Microsoft Exchange and Active Directory are an example of this. Gmail for business. Postfix with an LDAP server.
All modern email clients communicate with mail servers over TLS and will authenticate the server, just like a browser authenticates an HTTPS website. Modern email servers authenticate the client. What important security property do we get from the alternatives that we don't get from email? End-to-end encryption is the only one that occurs to me, and in an organization where we can trust our central server that's OK not to have. I suppose this is an easier proposition for companies than for loose groups of people organizing over the Internet. If you use a hosted email provider then obviously you must trust them, but self-hosted options are available and realistic (Exchange, Postfix).
Now, granted, what I am describing is a private organization rather than a federation. What's really cool though about email is that we get secure group communication within the organization, and also federated communication to other organizations on the Internet.
The security story for communication between organizations is weak, I'll grant. The primary weak link I see is not end user clients, but the communication between servers. Specifically, the lack of support in standards and servers for authentication of receiver by the sender, and for the receiver to declare that all incoming connections should come over TLS with a path-validated certificate. There is an Internet-Draft out for this called SMTP Strict Transport Security [1], but I don't know where it stands as far as support and adoption. DMARC+DKIM already provides authentication of the sender's organization by the receiver, and for declaring that all outbound messages from the sender's organization must carry a signature to be valid.
There's a fair amount of surface area in this model though: in particular, both end users and their mail servers need to be trusted. If in your communication between two organizations, you are willing to trust both organizations' central mail servers, then basic email setups will get you to a pretty good situation.
Some colleagues have recently made the case to me that S/MIME could fill in these gaps, and mentioned that folks in the industry are working to flesh out support for it. There are a number of problems to solve to make S/MIME practical, but with the will of a few major players I think we could get there. Some pieces that are missing include first-class client support, a key exchange/discovery/revocation system, and a scalable certificate issuance system. But solving S/MIME is only necessary to get to end-to-end encryption, and for most purposes it's practical to trust your organization's and partner organizations' email servers.
Even if email is not the secure group communication of choice, it's still used for a lot of secure communication and is worth improving IMO. I'm glad that Google and other have been investing in this: https://blog.google/products/gmail/making-email-safer-for-yo...