Why would you follow this guide?
What would your mail server do?
Your mail server can receive emails for an unlimited number of domains and users. The only limit is your disk space.
The server will accept and forward emails from your users to any other mail server on the internet. It will use encrypted connections whenever possible.
Power users will prefer to use a full-fledged mail programm like Thunderbird to read their emails. So your server will provide access through IMAP and POP3.
Your users just want a simple way to access their emails? With Roundcube they will need nothing but a web browser to access their inbox.
Keep most of the spam and phishing emails out of your inbox. And let your users configure their own mail filtering rules for mailing lists, auto-responders or to file common emails automatically on the server.
All outgoing email can be signed by the server using DKIM (domain keys). This will improve the deliverability and allow others to detect spoofing attempts.