OS X Lion 10.7 introduce a welcome enhancement to the legacy "Threads" view, with "Conversation View".
Conversation View condenses an entire ongoing email conversation by following the Subject line in your messages. For many this can be extraordinarily useful, and as a productivity tool is invaluable in speeding up your response times on emails.
There are however some important caveats to use that I've come across, and consider worth sharing.
Identifying Conversations in Your Mailboxes
You can identify a conversation in any given mailbox, by the little grey box with a number and triangle to the right of the email entry. The number naturally represents the number of emails within the conversation, and the triangle (a disclosure triangle) can be clicked on to reveal a list of the individual emails below. This is different than clicking on an email in the main list and seeing a full view of the emails one after the other in the preview window to the right.
Revealing the list of emails by way of the disclosure triangle can be particularly useful if you need to drag a single email within the conversation to another folder, but aside from that has little purpose in daily life.
Selecting an Email in Preparation for an Action
Aside from selecting emails through the disclosure method noted about, Often we need to perform an action on an email. Perhaps to relocate, flag or mark as read. In such a case, we can select individual emails from the preview on the right hand side.
By default, when you're replying to a conversation, Mail will assume that you're replying to the most recent message. This doesn't always fit the case, and when that need arises, you can click on any conversation email in the preview window, and the email will have a blue highlight against the grey background.
Now any action you take will affect that specific email message.
Conversations Traverse Mailboxes
What makes Conversation View work—and what's most confusing to people, is that emails within the conversation span the various mailboxes involved. If you've sent a reply to a friend or colleagues email, that reply will appear within the conversation—even though the email is within the Sent Mailbox.
In such a case, where an email physically resides outside of the inbox, its mailbox will be indicated under the timestamp on the right. Look for this if you're ever confused about why an email is where it is. This goes for any situation, not just pertaining to conversation view.
Gotchas to Watch Out For
The most common complaint that most people have about Conversation View comes about when multiple parties are CC'd in a backwards and forwards dialogue. It can be easy to overlook a second party's response to an email if the topmost and most recent reply exceeds the edge of the screen. If you didn't have the awareness to continue scrolling to see if the other parties had responded, earlier responses may get missed.
The moral of the story is that if you use conversation view, make sure you're scrolling past the most recent response to make sure you've read them all.
Turning Conversation View On & Off
When you first open Mail in OS X 10.7 & 10.8 (Lion and Mountain Lion) Conversation view is on by default. The first time you switch it off, the effect is global. After that, any changes you make to conversation view on a mailbox other than the main inbox, Mail will begin to treat each mailbox's conversation view status independently