Webmin gives you a convenient interface to read and write messages as any user of your system. This can be very useful when debugging mail problems, for instance, to check if a particular message reached a particular mailbox. Keep your users' privacy in mind of course.
Navigate to Servers | Read User Mail.
Click on the name of the user whose mailbox you would like to visit.
You will be presented with a graphical interface similar to any mail program, which allows you to read the user's e-mail messages:
Click the Compose button to see an interface for writing and sending a new message.
Enter a destination address, message subject, and body.
Click the Send Mail button.