About this osTicket Installation
Server Information
osTicket Version v1.15.5 (2ba1d8e) — Up to date
Web Server Software Apache
MySQL Version 5.7.33
PHP Version 7.4.16
PHP Extensions
gdlib Used for image manipulation and PDF printing
imap Used for email fetching
xml XML API
xml-dom Used for HTML email processing
json Improves performance creating and processing JSON
mbstring Highly recommended for non western european language content
phar Highly recommended for plugins and language packs
intl Highly recommended for non western european language content
fileinfo Used to detect file types for uploads
zip Used for ticket and task exporting
APCu Improves overall performance
Zend Opcache Improves overall performance
PHP Settings
cgi.fix_pathinfo "1" is recommended if AJAX is not working
date.timezone Setting default time zone is highly recommended
Database Information and Usage
Schema helpdesk (localhost)
Schema Signature add628927ee030469f5d3272ebda1e16
Space Used 77.92 MiB
Space for Attachments 8.52 MiB
Timezone GMT (Interpreted as Europe/London)

Installed Language Packs
English (United Kingdom) — en_GB
en_GB — include/i18n/en_GB.phar
Version: 154948, for version v1.10
Built: Wed, 06 Feb 19 21:17:47 +0000
English (United States)
en_US — include/i18n/en_US

Scenario - a customer logs a ticket by sending an email to one of the helpdesk's email addresses. It is picked up by a cron job and a new ticket is created. An agent replies and an email is sent to the customer. All so far so good.
If the customer then simply hits 'reply' to reply back to the agent's answer mail, a new ticket is created, rather than the reply being added to the existing ticket thread.

I'm not sure, but I think this behaviour has started to appear since we upgraded to 1.15.x.

Is there something I need to check that has changed from previous versions to make the expected behaviour (all replies attach automatically to the existing thread) happen?

Many thanks

Steve