In OEM 12c Enterprise Manager, you can change both the body and the subject of email alerts through the web-based EM console.
Once logged in, you should see menu options in the top right corner like "Setup" and "Help" with down arrows indicating a drop-down menu.
Click "Setup" -> "Notifications" -> "Customize Email Formats..."
From here, you can select the type of email that is being sent (Metric Alerts, Job Status Changes, etc.), the format (Long or Short), and you can also customize the subject and body of the email by clicking "Customize". A sample email based on your changes will be shown below the "Customize" button.
When customizing an email format, you can add plain text in both the subject and body, and you can also use the predefined labels and attributes (links to these are on this same page) to include dynamic information based on the type of alert and other status values.
Once you save this, your email notifications from OEM 12c should reflect whatever changes you made.
In addition, if you go to "Setup" -> "Notifications" -> "Notification Methods", you can change the name that appears as the sender of the email (e.g., "OEM 12c Administrator") as well as the email address that the alert appears to be sent from. This also allows you to use rules and filtering on the email alerts by filtering on the "FROM:" email address and sender name.