The reason is because Spam Assassin or another program such which uses the same lists as Spam Assassin, have added a new policy.
It seems a lot of setup guides show that you should remove certain lines from all emails that pass through your servers. One of these lines is the "Received" line which shows the IP of which the email client that sent out the email.
Fighting spam is a hard battle, and leaving in the received header makes a lot of sense to target those actors.
Remove the following line from your header_checks file, under your postfix installation directory.
Remove this line:
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