Herald for Mac allows you to customize quite a few options related to the notifications you receive for e-mail. While the newest versions of OS X have included quite a few notification upgrades, Herald allows you to do much more, customizing the transparency, colors, fonts, and layout of the notification boxes that appear when you receive new e-mail on your computer.
To install Herald you'll need to first download and unzip the file. It is installed as a plugin for the tools included in your Mail's Preferences menu so that is where you will find the options. You can then change when the notifications appear, what they look like, which mailboxes you receive notifications for, the sound that plays when you receive e-mail and quite a bit more. Everything is very clearly laid out in a Mac-style interface, and the entire process takes only a few minutes to set up and seconds to customize in the future if you want to make changes. Combined with the tools, you get built-in with Mac OS X 10.9, this is a great plugin that makes it a lot easier to see what messages you are receiving and whether they deserve an instant response.
If you are looking for a more powerful plugin for your Mac's Mail app or if you want to customize the way those notifications appear when you do get mail, then consider Herald for Mac. The app is completely free, updated for Mavericks, and is a fantastic way to get more information without slowing down your productivity on a Mac.