A chatmail relay is a standard email server you'd find anywhere else with some specificities:

• any account is valid. To "create" an account just login with it and your password in your usual mail app and the account will be created. Of course if it already exists you must have the correct password, so you can't "steal" an account
• messages are automatically and unconditionally deleted after a certain amount of time. Inactive accounts are deleted after a certain amount of time. Each relay publishes their setting, I think the default is something like 30 and 90 days, respectively
• incoming and outgoing messages must be pgp-encrypted. Any plaintext message will be dropped
• Only dkim is setup, other mechanisms aren't so you might have trouble with some of the stricter checks of other servers

A chatmail relayis just a box to receive and relay messages for the deltachat apps, but since it uses completely standard protocols it interacts with the rest of the email ecosystem