ETHMail.cc is a email service provider, it was created to allow anyone with an email address to contact a person owning certain Ethereum address even when it's the only thing they know about this person.
For a person who want's to send email to Ethereum address there are no special conditions to be meet. Anyone with an email (e.g. gmail) account can send messages to 0xsomeethereumaddress@ethmail.cc
For owners of ethereum addresses to read the message that was sent to them they need to either go to web.ethmail.cc and login using their ethereum wallet (it involves signing a login message to prove ownership of given address) to see their inbox or setup their email app (like Gmail on mobile or Thunderbird on desktop) to receive messages from ethmail.cc servers
ETHMail.cc in it's current form works exactly like any small email provider you can find on the internet. It runs servers that talk over SMTP and IMAP protocols to communicate with other email servers and with email client applications. The difference is that it provides mailboxes for any ethereum address in existence.
ETHMail.cc provides support for ENS and .crypto domain based email addresses.
If you have ENS name with reverse lookup setup you are able to use [ENS name]@ethmail.cc email address (e.g. mine is xukulapchvatal@ethmail.cc because I own xunkulapchvatal.eth)
If you have .crypto domain you are able to use [.crypto domain]@unstoppable.email email address.
Those addresses are currently only serving as "routing" layer. The messages are directed to mailboxes associated with ethereum address owning them. Each ethereum address have one dedicated mailbox.
There is an ongoing conversation at https://community.cryptoverse.cc/ about dedicated mailboxes for NFT tokens which could be expanded to create dedicated mailboxes for ENS or .crypto domains
Currently there are available 2 plans you can use with ETHMail.cc: Free and Pro
Free plan is automatically available for everyone and enabled by default. It's provides basic mailbox functionality so you can receive and send messages with it, but it has limited capacity and number of messages you can send. Messages stored in free mailboxes will be removed after some time, so be sure to either download them to your client app or switch to Pro plan.