Customer does not receive email notification when agent replies to ticket via email

[pandafy]

Information about bug

Steps to reproduce:

1. Customer sends an email to the helpdesk address (e.g. helpdesk@example.com).
2. Ticket is created in Frappe Helpdesk.
3. Agent receives an email with headers:

From: Customer <helpdesk@exampe.com>
X-Google-Original-From: Customer <customer@gmail.com>
...other headers omitted for brevity

4. Agent replies to this email from their inbox (e.g. Gmail).
5. Reply appears in the ticket timeline in Frappe Helpdesk UI.
6. Assigned agents receive notification about new comment.

Actual Result:
Customer does NOT receive an email notification about the agent's reply.

Expected Result:
Customer should receive an email notification when an agent replies to their ticket.

Reply email sample header:

From: Gagan Deep <agent@example.com>
To: helpdesk@example.com
...other headers omitted for brevity

Version

Frappe Framework: v15.101.1
Telephony: v0.0.1
Helpdesk: v1.21.1

Installation method

No response

Relevant log output / Stack trace / Full Error Message.