Every time you receive an email in Outlook you get a notification that pops up with the summary information for the email. You can configure Outlook on how these notifications are presented: you can adjust how long they are shown, their transparency, if they include a noise, or you can turn them off entirely. It's not hard to adjust, but in true Microsoft fashion it is pretty well hidden. Here's are the steps to adjust the notifications.

1 - From the menu select Tools -> Options.

2 - Make sure you are on the Preferences tab.

3 - In the E-mail section (at the very top), click on the E-mail Options... button.

4 - On the next panel, click the Advanced E-Mail Options... button in the Message Handling section.

5 - In the When new items arrive in my Inbox section you can uncheck the boxes next to Play a sound and Display a New Mail Desktop Alert to turn off the alerts entirely. If you want to adjust the length of time the notifications are shown or their opacity click the Desktop Alert Settings... button.

6 - You can then adjust the transparency and the display time.

Once you make your changes, click the OK buttons as needed to save your changes.