ISSUE DESCRIPTION :

The user sent an email from outlook to customer(If the customer having the contact/account/Lead record in CRM), then the sent emails are not tracked automatically into CRM from outlook desktop sent item folder.

CAUSE:

To specify whether to track all email messages or only certain types :
Email messages from Dynamics 365 apps Leads, Contacts, and Accounts. Tracks email messages only if they originate from someone with a Dynamics 365 for Customer Engagement lead, contact, or account record.

More information: OrgDBOrgSettings tool for Microsoft Dynamics CRM
AutoTrackSentFolderItems : False : Setting value to TRUE will result in Server Side Sync auto tracking of emails from Sent Items. This setting only applies if the mailbox is configured to track "All Email Messages"

Default value is set to False to preserve the existing behavior.
To enable functionality on the organization "AutoTrackSentFolderItems" should be set to True.

Work around :
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You can track the email manually by using app for outlook

Ask :
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The user sent an email from outlook to customer(If the customer having the contact/account/Lead record in CRM), then the sent emails are should tracked automatically into CRM from outlook desktop sent item folder.