Setting the Regarding Object for an E-Mail should be the same dropdown in the Mail Inbox as in the E-Mail Detail Form
Setting the Regarding Object for an E-Mail is an important task in the CRM Outlook Client.
In the E-Mail Detail Form you have a dropdown in the ribbon, with the latest used regarding objects to set as a reference for the E-Mail.
In the Mail Inbox you don't have this dropdown behaviou...
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Declined