How to Group Messages by Thread in Mac Mail
An email discussion that spreads across dozens of messages in your inbox is confusing to follow. The Apple Mail application in macOS and OS X prevents this by grouping emails in conversations (threads) instead of showing each email separately.
You can turn off conversations or turn them back on easily for your Inbox or any other folder in Mail. To read your messages organized by a thread in any folder:
Open the inbox or folder in which you want to read mail organized by thread. The Mail application remembers your choice for each folder, so changing the setting on one folder doesn’t affect any other folder. Select View > Organize by Conversation from the Mail menu bar. If Organize by Conversation has a check mark next to it, threading is enabled. If not, click it to place a check mark and enable conversations. Make conversations stand out in your inbox by selecting View > Highlight Conversations in the Mail menu bar.
How to Work With Conversations in Mail
Only the newest message in a conversation shows in your email list by default—unless you request the oldest message in the drop-down menu above the list of emails or in Mail preferences.
To expand a single conversation and view all the emails in it, click the message as you would any other. When you expand a conversation, all the emails are visible in the reading pane. To expand all conversations rather than a single one, click View > Expand All Conversations in the Mail menu bar. When you don’t want to see them any longer, click View > Collapse All Conversations. Press Option+Up Arrow or Option+Down Arrow to move through the emails in a conversation quickly.
Preference Settings for Mail Conversation View
To pick conversation view settings that work for you in Mail:
Select Mail > Preferences from the menu bar in the Mail application. Go to the Viewing tab of the Mail preferences. Check Include related messages to have Mail find messages in the same thread from folders other than the current one and insert them into the thread where appropriate: Emails from other folders, such as Sent, are not listed in the message list but appear in the reading pane’s full thread view. You can still reply to, move, or delete these messages. Related messages list the folder in which they are located. To change the order in which emails show in the reading pane’s conversation view, check Show most recent message at the top for reverse chronological ordering and uncheck it to view emails in chronological order with the oldest first. Check Mark all messages as read when opening a conversation to have all emails in a thread marked seen as soon as you open the thread in the reading pane. Close the Viewing settings window to save your preferences.
How to Disable Grouping by Thread in macOS Mail and OS X Mail
To turn off conversation grouping in macOS Mail:
Emails from other folders, such as Sent, are not listed in the message list but appear in the reading pane’s full thread view. You can still reply to, move, or delete these messages. Related messages list the folder in which they are located.
Open the Mail application on your Mac. Go to the folder for which you want to disable conversation view. Click View in the menu bar. Click Organize by Conversation to remove the check mark next to the option.
How to Group Messages by Thread in Early Versions of Mail
The process of grouping messages by thread is slightly different in versions of Mac OS X Mail 1 through 4. To browse your mail organized by thread:
Open Mail on your Mac. Select View in the menu bar. Click Organize by Thread to place a check mark next to the option.
If you want to turn off this feature, return to View on the menu bar and click Organize by Thread to uncheck the option.