Airmail allows you to arrange the messages by subject, which will in turn convert them into conversation view. However, the replies of conversation will only be included if the Subject has not been changed. 

The action of this feature will only be performed within Airmail and this will not the server message list or other email clients using the same account. There is an option in Preferences > Appearance as shown in the picture below. Please note that you have to quit and restart the app to see the results.

Group by Subject option will affect entire email accounts that is listed on Airmail.

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