By default, Mail groups messages that are part of the same “conversation” (or thread) in the message list, rather than listing each reply individually. Viewing messages by conversation makes it easier to follow a thread and helps streamline the message list.
Turn conversations on or off: Choose View > Organize by Conversation (a checkmark shows it’s on).
Expand or collapse conversations in the message list: For one conversation, click the message count in the top message. For all conversations, choose View > Expand All Conversations or View > Collapse All Conversations.
See the subject line in the preview area: Scroll to the top of the preview area—the subject line is shown there instead of in each message. You can also click Details in the header area of a message, or look at the subject line in the message list.
Step through a conversation in the preview area: Press Option-Up Arrow or Option-Down Arrow.
Show more of a message: If a reply contains the original message but doesn’t show all of it, click the See More link at the bottom of the reply.
Change the order of messages in the preview area: Choose Mail > Preferences, click Viewing, then select or deselect “Show most recent message at the top.”
Move messages in a conversation: When you move a message to another mailbox, it becomes a related message, but is still shown in the conversation.
Hide related messages located in other mailboxes: Choose Mail > Preferences, click Viewing, then deselect “Include related messages.” To hide them only until you quit Mail, choose View > Hide Related Messages.
Delete a conversation: Select a conversation in the message list, then press Delete. All of the messages in the conversation are deleted, except for related messages located in other mailboxes.
When messages aren’t grouped into conversations, and you select one of the messages in the message list, the other messages are highlighted, so you can still identify the thread. To change the highlight color or turn off the highlighting, choose Mail > Preferences, then click Viewing. If you want to see the highlighted messages grouped together in the preview area, choose View > Show Related Messages.