I need to host 3 domains for manage the emails. I can´t found documentation about this.
I can work with multiples domains for send and receive emails? I will to create a new nextcloud server and iḿ thinking to point all dns mail configurations for this new server,
I want to terminate my postfix and only use nextcloud in my life. This is possbile with multiples domains?
The Nextcloud Mail Apps are mere mail clients, supporting multiple accounts, indeed, but neither intended nor technically empowered to act as mail servers/MTAs. This should also answer your other question, at least partially.
Complete mail servers you might use include, but are not limited to:
Mailcow
Mail-in-a-box
Mailu
Any of these will basically recommend having their own dedicated box. Should help you find a solution, which you can then integrate into your existing Nextcloud.