CRM Tracking Token

Once I received an email from client A, and I set this email as Track in CRM in the outlook.

Then I replied this email.

So both incoming and outgoing email can be found in the CRM system.

Outgoing email has a tracking token "CRM: XXXXXXXXX", but the first email (incoming email) does not have the token.

Is there anyone tell why?

Thank you,

Keiutou