Notable: Herald is no longer supported on macOS 10.14 and later. Herald 8.0.2 for macOS 10.13 is the latest supported version. Versions that work on 10.12 and earlier versions of Herald are available at the Herald website.
Herald, a notification plugin for Mail.app is Apple's macOS email client. Herald is a plugin that displays popup notifications when you receive new Mail. You can also perform common actions from within the notification window. This includes deleting messages, initiating replies and viewing Mail messages. Herald's appearance can be customized by changing the background color, text color and size, transparency, transparency, and position. Herald also offers a scrolling text view that allows you to read all messages without switching back to Mail. Herald is available for free and requires macOS Sierra. Apple does not officially support Mail plugins. Although Herald has been thoroughly tested it is possible that it could cause unintended conflicts between Mail and other unofficial Mail plugins. This is especially true after OS updates. It is recommended that you back up your computer before installing Herald, or any other Mail plugin. Also, it is recommended that you back up your computer before installing any updates to Mail and macOS.
The software is freeware but it is donationware. To support development, please consider donating.