First create a user account (or use a builtin account):
Virtualmin>Edit users:
Add a user:
Email address: [email protected]
POP3 login username is automatically chosen for you. Set the password.
You can test mail sending by logging in at the webmail interface at https://gody.com:20000/?mail

Next add a mail server record:
Webmin>Servers>Bind DNS Servers>
Choose your domain by clicking on it:
Name is the website name typically.
Eg: I want to send mail to [email protected], fill in the following details:
Name gody.com
Mail Server: mail.gody.com
TTL: Default
Priority: 5

Now go to your DNS registrar (Eg Cloudflare) and setup the following records:
Add an MX record.
Name: gody.com
Value: mail.gody.com
TTL 2 minutes

Now test your settings in a POP3 program like Outlook:
Account type: POP3
Incoming mail server: mail.gody.com
Outgoing mail server: mail.gody.com
Username: joel.gody
Password: What you chose.