ETHMail.cc run email servers in the cloud and is configured to accept email messages for email addresses that consist of valid ethereum address, ENS name or .crypto domain.
When someone want to send and email to 0x123@ethmail.cc from their e.g. gmail account all they need to do is simply create this message in gmail interface. After that gmail servers contact ethmail servers using SMTP protocol and ethmail check if the username part of the email address is valid. If it is, then the server accepts the message and stores it in place dedicated to this ethereum address.
Later when a person owning this 0x123 address wants to read messages left for them, they can login to ethmail web interface which will show their stored messages or configure their email app to automatically download messages from ethmail servers and notify them when any message arrives.
There is no automatic encryption in ETHMail yet, but this topic comes back quite often and I decided to describe possible solutions here. I'm unsure which of the solutions will be implemented and when
ETHMail is designed to work with classical email systems allowing people outside crypto space to send messages to people "inside" when all they know about them is their wallet address, and the other way around. It is also a principle in cryptoverse products to not reinvent the wheal and allow people to use already existing software (e.g. email apps).
This poses some challenges to automatically encrypting all communication between participants.
In other words - it's impossible. There are some exceptions through.
When someone decides to send email from e.g. Gmail to ETHMail the message first goes through Gmail UI and servers and only later reaches ETHMail servers.
In this scenario the sender is responsible for encrypting a message but Gmail or other email providers do not provide automatic message encryption. You can use extensions like mailvelope that use PGP but you need to have recipient PGP public key to encrypt the message (ethereum address in not enough, it's not even a full public key) - So this is our first challenge, if someone wants to communicate "securely" (mostly, somewhat, not really - SMTP leaks metadata all over the place, the only thing that is encrypted is message content) using exiting tools they need to know PGP public key.