Wikipedia
NeXTMail
NeXTMail was the email client for the NeXTSTEP operating system. It was one of the first e-mail clients to support universally visible, clickable embedded graphics and audio within e-mail. NeXTMail made attaching images, text and videos to a message straightforward using drag and drop. It featured a service named "lip service" that allowed a user to click a button and record audio. NeXTMail evolved into Apple Mail, the e-mail application bundled with Mac OS X and iOS. Steve Jobs continued using NeXTMail after his return to Apple, until OS X grew stable enough for every day use.
NeXTMail is said to have been inspired by Nathaniel Borenstein's mail system.