As the topic name says, it appears content after a horizontal line in email replies is stripped away, even though the horizontal line is in a codeblock.
See Difference between `fail` and `fail-check`? - #3 by sorawee - Questions & Answers - Racket Discussions for an example. It’s supposed to look like this:
pre-content
@--------------------
post-content
pre-content
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post-content
But Discourse incorrectly processed it, and showed:
pre-content
@--------------------
post-content
pre-content