The Mail set-up in System Preferences can also be reached from the Mail.app preferences.
To access your mail, launch Mail.app. In the left navigation of Mail,app, you can click on the disclosure triangle next to the Inbox entry, and can then select and view each mail server's inbox individually.
Click on the Inbox at the top of that selection to view all inboxes together.
By control-clicking on a specific mail server listed under the main Inbox of Mail.app, you can also take that mail account on or offline using the pop-up.
And since you're connected to multiple mail servers... When sending mail, you can control which mail server is used in the compose window, if you customize it. To customize the compose window, select File > New Message (or press ⌘N) and then select the button with the three lines and the triangle to the left of the compose window. If you hover the mouse pointer, it'll tell you this is how you customize the display. Press the button, and select Customize from the popup. Then (if you want, and if it's not already selected) click the checkbox to left of the From selector, or the mail server selector to the right of the From selector.) Then press OK.