How to Thread E-Mail Messages with Your iPhone 4
As part of the iOS 4 upgrade, Apple lets you thread e-mail messages on your iPhone 4, meaning Mail automatically groups related missives. When you organize e-mail messages by a thread, the related messages show up as a single entry in the mailbox, with a number next to the right-pointing arrow in the entry indicating how many underlying messages are represented by the thread.
You first have to turn on threading in Settings:
From the Home screen, tap Settings→Mail, Contacts, Calendars→Brganize by Thread.
Tap the Organize By Thread button so that it reads On.
You may have to scroll down to see the Organize by Thread setting.
After you've turned on Organize by Thread, you can access a thread in any of your inboxes:
Tap the Mail icon on the Home screen.
The Mailboxes screen appears. At top of the Inboxes section is the All Inboxes inbox, which, as its name suggests, is a repository for all the messages across all your accounts.
Tap the folder you want to open.
Tapping All Inboxes displays all your messages from every e-mail inbox you've set up. In the All Inboxes inbox shown here, the number 4 next to Teresa's message shows that four messages constitute this particular thread.
Tap the listing whose thread you want to see.
The threaded messages for that listing appear. In this example, the screen displays all four threaded messages.
When you look at a message that's part of a thread, the numbers at the top of the screen tell you your location in the conversation. For example, in this figure, the note is number 3 of 4 in this thread.