In user settings you have option E-mail access Type incoming and you can select Outlook or Exchange Email Router. You are not able to choose multiple options. If you want to track all messeges (even not related to work) I suggest Email Router because it is working good (sometimes only it cannot track single email from various reason like special characters in subject etc.). Also Email Router is only option to catch emails if users are on WebClient.
The reasons between Outlook Client and Web Client are mainly related to calendar and promote items from Outlook store (emails,tasks,appointments,contacts) to CRM and synchronize.
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