There are several entries in the forum about notifications or alerts not being sent to the assigned Agent when a user has replied to a ticket. I've gone through all of the forum topics about it but found no solution to the problem I'm seeing since we updated osTicket from 1.09 to 1.18.

Since I was uncertain to which exact existing topic this would fit in, I thought to start a new topic as I might also have found the root cause. I will describe.

The problem:
When a user replies to a ticket via email, the reply is correctly added to the ticket, but most of the times no alert is sent to the assigned Agent, so the agent is not aware that a reply was received.

Tests done
I've done extensive testing and have found out that it has to do with the email client the user is using to send out their reply. I created a ticket for a test user being myself (with a @live.nl email address, not being the addresses used within the ticket system whatsoever!). That user is "Donald Duck". The user created a ticket and I answer to the ticket as the Agent in osTicket. User receives the answer. When user replies via email, using a simple email app on the Android phone (Bluemail in this case) the reply is correctly received in osTicket AND an alert it sent to the Agent.
However, when the user replies from the SAME @live.nl email address configured in MS Outlook, the reply is again correctly added to the ticket BUT NO ALERT is sent to the agent.
When the user replies using the link in the bottom of any ticket response, so not by sending an email but directly entering it into osTicket, the alert is also successfully sent to the agent.

I've already had a close look at the email headers of both emails and could not find any entries in the headers as listed in class.filter.php lines 903-915 such as suggested by @KevinTheJedi in several Forum topics. So, I'm in the blind about why the reply from user sent from Outlook is not triggering an alert to the agent while it does when the email is sent with another email client. I will attach both headers as replies hereunder (I have anonymized the email addresses within them). (not sure how to upload these, forum rejects .TXT files are not allowed, so I posted them as replies, hope that is ok)

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