For every incoming email, Freshdesk checks 3 email markers to understand whether the email is a reply to an already-existing ticket. If the email is identified as a reply, Freshdesk performs additional sender email checks to make sure it's not spam.
Email Markers Check
Freshdesk looks for the following three markers in every incoming email. For an email to be threaded to a ticket, it has to pass one of the following checks:
- Ticket ID (if enabled)
- Message-ID
- Unique Identifier
For detailed information on ticket threading in Freshdesk, please refer to the article here