This series in intended to be followed in chronological order. This article assumes you have followed the previous articles in this series.
If emails being sent using Postfix are arriving at a clients junk, this could be due to the client email software flagging the email as spam due to certain keywords in the subject or body of the email. As an example, I experienced emails arriving in a clients junk when the subject was test and the body of the email was super test.
On the other hand, when I sent the email where the subject was jpg attached and the body was our cats, the email arrived in the clients inbox. So, when emails being sent using Postfix are arriving in a clients inbox, the first thing to do is to try to modify the text in the subject or body to determine if the clients email software is flagging the email as spam due to certain keywords.