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.
From Erik Hinterbichler:
Herald is a notification plugin for Mail. app, Apple's Mac OS X email client. Inspired by the excellent Mail. appetizer plugin, Herald shows you popup notifications whenever you receive new Mail. It also allows you to perform common actions directly from within the notification window, including deleting messages, initiating replies, viewing messages in Mail, or marking messages as read. Herald's appearance is highly customizable, allowing you to change the background color, text color, size, shadow, transparency, and position of the window. Herald even provides a scrolling text view so you can read full messages without having to switch back to Mail. Note: Mail plugins are not officially supported by Apple. While Herald has been thoroughly tested, it's possible it may cause unintended conflicts with Mail or with other unofficial Mail plugins, especially after OS updates. It's recommended that you backup your computer before installing Herald or any other Mail plugin, and always before installing any updates to Mail or OS X.
You can evaluate each message as it arrives, and respond/delete/ignore as you like. Very easy to customize, and the Developer updates within days of each new Mac version.
It freezes the Mail application when Herald is loading new messages-the only con I can think of, and it's Pros heavily outweigh this one annoyance. I've communicated with the Developer about this--he is aware and says he can't solve the issue yet.
Solid and inobtrusive. Rapid updates when MacOS or Mail.app updates break it.
Does what it is supposed to do. Allows me to do many other things on my Mac and be alerted when new mail comes in but is still in the background. The summaries allow me to disregard unimportant messages and keep working on whatever I was doing, or take care of business immediately.
It never requires opening like some apps. I can find out what I need to know without having to open Mail and then going through a procedure to return to what I was doing.
It can be distracting when I am concentrating on a project and won't go away without clicking. When I receive multiple eMails at once, I have to keep clicking to get rid of them. Better that than having to remember to turn Herald on.
One of the best applications I have downloaded. And it is free. I have not encountered any glitches or conflicts.