When you turn on message threading, Mail will collect messages with like subjects into groups. Mail sometimes gets this wrong, but the feature is still worth very useful. You’ll get a bunch of new UI, such as coloring, indenting and faux messages once you turn this on. The Faux message will be one that is a summary of the entire thread. You can click on one of messages in the thread to jump to it.
However, the coolest part of message threading (to me) is keyboard navigation. When a thread header faux message is selected you can:
- Press Delete to delete the entire thread
- Press down arrow to skip over the thread
- Press right arrow to expand the thread and select the first unread message (or last message in the thread if all have been read)
- Press down arrow to read the next message
- Press left arrow to collapse the thread