As per the Salesforce Spring '18 Release Notes -
Prevent users from getting read receipts for email messages they send. Read receipts include information such as when the recipient opens the message and from where. Laws and regulations can require you to honor and respect your customers’ wishes regarding the personal data you collect.
Here are a couple laws that are important to many companies collecting and processing their customers’ data:
General Data Protection Regulation (GDPR), European Union
Gramm-Leach-Bliley Act (GLB Act), United States
To prevent users from getting read receipts, from Setup, enter Inbox in the Quick Find box, then select Setup Assistant. Toggle the Send Read Receipts setting to Disabled.
This means the Salesforce Inbox tracking feature may be against the GDPR compliance which could be turned off by using the above method.
For more visit this link - Prevent Read Receipts to Enhance Privacy