This also covered in the documentation.
docs.microsoft.com/.../email-message-filtering-correlation
Server-side synchronization, Microsoft Dynamics 365 for Outlook, or the Email Router can automatically create email activities in model-driven apps in Dynamics 365, such as Dynamics 365 Sales and Customer Service, which are based on received email messages. This type of automation is known as email message tracking. Users can select a filtering option that determines what email messages will be tracked. Filtering is set on the Email tab of the Set Personal Options dialog box in the client applications. Users can set the following options:
All email messages. All email messages received by the user are tracked (will have activities created).
Email messages in response to email. Only replies to email messages that have already been tracked will be saved as email activities. This option uses smart matching, a correlation method that uses the existing properties contained in the email to relate email messages to activities.
Email messages from Leads, Contacts, and Accounts. Only email messages sent from leads, contacts, and accounts in the database are saved as activities.
Email messages from records that are email enabled. Email messages are tracked from any record type that contains an email address, including customized record types (entities).
By default, the Email messages in response to email option is enabled. Correlation occurs after an email message is filtered. System administrators can turn off all message tracking for a particular user by setting Incoming Email under Synchronization Method to None on the Mailbox form.
Email correlation is set on the Email tab of the System Settings page and can be enabled or disabled for the entire organization. Model-driven apps in Dynamics 365 uses two kinds of correlation, tracking tokens and smart matching. By default, both correlation types are enabled.
