Email Threading on an iOS machine
TheApple iOSbuilt-in Mail utility has several tweaks controlling email threading.
Email threading is turned on by default.
Go toSettings, and selectMail.

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You have the following options to choose from:
Toggle on any or all of the options to enable.
Scroll down past all your folders and selectSettings.
pick the checkbox next toConversation View.

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Return to your email to view your threaded email conversations.
SelectOptionsfrom the context right pane that displays.
Use the toggleShow messages arranged by conversationsto turn this option off or on.

This setting can be controlled for each email account you set up in the Mail app.