One line in bash will do it. You're probably on Windows... if you are, install VMWare workstation and a Linux distro or enable WSL 2 on Windows, download Windows Terminal or use cmd.exe to enter wsl, windows subsystem for linux.
bash prompt:
┌──(throwawayz㉿suffer)-[/home/throwawayz]
└─$ < LIST.txt grep -Eo '\b[a-zA-Z0-9.-]+@[a-zA-Z0-9.-]+\.[a-zA-Z0-9.-]+\b' | grep -Eo '[^@]+$' | sort -u
output.com
"< LIST.txt grep -Eo '\b[a-zA-Z0-9.-]+@[a-zA-Z0-9.-]+\.[a-zA-Z0-9.-]+\b' | grep -Eo '[^@]+$' | sort -u"
^ this command,
feeds the original file named LIST.txt in this example to grep, grep uses regex to filter and pull only what follows @ and matches the accepted format for a domain.