It sounds like the email address associated with the helpdesk tickets might not be set up to properly handle email replies. Here are some things you can check:

Make sure the email address associated with the helpdesk tickets is set up as an alias that forwards to the helpdesk email address in Odoo. This way, any email replies will go to the helpdesk email address and be associated with the existing ticket.

Check that the "Inbound Mail" settings in Odoo are set up correctly. Make sure that the "Alias domain" is set to the domain that the helpdesk email address is using, and that the "Server Type" is set to "IMAP".

Double-check that the "Reply-To" address for the helpdesk email address is set correctly. This address should be the same as the "Alias domain" you set in step 2.

Make sure that the email replies are being sent from the same email address that the sender originally used to create the ticket. If they are using a different email address, Odoo may not recognize the email as associated with the existing ticket.

If you've checked all of these settings and the problem persists, it may be worth reaching out to Odoo support for further assistance.

Is This problem resolved? I am also facing the same issue while using AWS SES email credentials configured as outgoing mail server.