Weird thing happened with my mail.app that I want it to do again

Question

a couple of days ago, I ws using my mail.app and I looked at my messages and there was one message from amazon.com when there should have been 3. It said I have 3 messages from amazon.com & when I clicked on it the 3 messages appeared like they were filed away there. I want to do it again or at least figure out what it was

Answer 1

This is called organize messages by thread.
The following was copied from Mail Help:
A "message thread" is the chain of replies in an email conversation. Mail can help you quickly identify all of the messages in a thread by highlighting them in the color of your choice and grouping them together under the most recent message, sorted by date. Even if someone changes the subject line, Mail keeps the messages grouped together with the other messages in the thread. You can view them together and file them or delete them quickly.
To highlight all messages in a thread in the same color when you select one message in the thread, choose Mail > Preferences and click Viewing. Select "Highlight related messages using color."
To select a highlight color, click the color well in the Viewing pane of Mail
Preferences and choose a highlight color in the Colors panel.
To group your email by thread, select a mailbox and choose View > "Organize by Thread."
To see all the messages in all threads, choose View > Expand All Threads.
To see all the messages in a specific thread, click the triangle to the left of the thread or select the thread and press the Right Arrow key.
To collapse all threads, choose View > Collapse All Threads.
To collapse a single thread, click the triangle to the left of the thread or select the thread and press the Left Arrow key.
To step through messages in a thread when you have organized your mailbox by thread, choose View > Select > "Next Message in this Thread" or "Previous Message in this Thread." Or, use the Up Arrow and Down Arrow keys.
When you select a collapsed thread, the message content area of the Mail window shows the subject of the thread, the originator of the thread, the date the thread was started, the number of unread messages, the total number of messages, and a list of the messages, including which ones came with attachments.
Mail uses special headers in each email message to determine if a message belongs to the same thread. If these headers are not present, the subject is used to determine if a message belongs to the thread.