The dash lines must be in the content of your messages -- *some* messages, I would presume -- but maybe if you get "similar enough" messages all the time, then it looks like a pattern to you...
PS - and they are, both messages with those lines are from Weekly Recipes. And there is a message from a different sender that does not have that line.
PPS - the line is in the "plain text" parts of those messages -- which Aqua uses to extract previews, to avoid having to parse HTML.
When you open those, if there is an HTML part too ("alternative") then that's what you'll see.
But that doesn't in any way negate that those messages do have dash lines (in their "text/plain" format).
