How are end-to-end encrypted messengers protected against man-in-the-middle attacks?

Question

Encryption

Let's say some company released a messenger, declared that the messenger uses end-to-end, encryption, and even published the source code so that "especially smart" users could make sure that there are no backdoors.
But after all, all traffic goes through the company's servers, and nothing prevents them from organizing a meeting. Let's say this is a decent company, and they don't do that, but there is a non-zero chance that the servers will be compromised.
What are the opportunities to protect yourself?

Answer 1

None. It's purely a matter of trust.
Even a client placed in the market may not be the one posted in the source code at all. Even if you compile the client from the sources, an update will arrive, without which the messenger will refuse to start - that's all.
If the traffic passes through someone's servers, it is best to consider that it is viewed by everyone who is not lazy.

Answer 2

Use another communication channel, exchange public keys and then work out a shared secret with some Diffie-Hellman. Something like this.

Answer 3

If the messenger is for android, then there is a special. software for checking open, pending and application-established connections for protection from the mitm-vector of the key exchange and connection process itself (!!! But mitm is such a thing a vector one step left / right and the attack is already rolling, so the protection of software cascades taken separately is useless (IMHO) ..
PS: applications can be found in the google play market at the request "mitm"; "mit test".

Answer 4

No, it's a matter of trust in the company. Raise your server, the so-called self-hosted, such as MyChat, Jabber or something like that - then you will have some kind of guarantee, and then only because you can control the traffic between the server and the client.

Answer 5

RetroShare easily breaks through NAT
Tox messenger easily breaks through double NAT
XMPP with OTR
JAMI is still damp but you can use
all of the above is open source I personally tested all these programs