Q:

What is this blue line in Outlook, and why does it behave so strangely?

I'm using Outlook 2007. I do not have Word installed. I think Word replaces Outlook's message editor if you have both installed, so take care when answering that you're talking about the same thing.
When I forward an email, Outlook inserts a blue line above the forwarded email:

What is this line? It's not the same thing as a creating a "horizontal line" or a "border" from the paragraph section of the ribbon. Here you can see it compared with them:

I originally thought it could not be deleted, but this is not the case! It can be deleted, but it seems only to be deleted when I delete the paragraph break after Subject. If I insert any paragraph breaks between the line and the "Subject" paragraph break, it won't go away until I delete all of those paragraph breaks.
It can't be copied and pasted, even if you select the entire document. The pasted copy simply doesn't have the line anymore.
If I select the line of text with "Subject" on it and click "clear formatting", the blue line disappears. If I undo this, the blue line does not return, just as if it had never been there!
Here's the blue line when you put it next to bottom and top borders. It fits snugly against the bottom border above it, and with a gap between it and the top border below:

What is this weird line, and why does it behave so strangely? Is there some way of creating one of these normally? (Not that you'd want to!) Is it some artefact of representing a richer underlying document model through the limited features of Outlook's editor?

A:

It is a border, just styled. Save the email as an html file and you can see it in the source.
<div style='border:none;border-top:solid #B5C4DF 1.0pt;padding:3.0pt 0in 0in 0in'>

It is the top border of the <div> section that represents the previous email's header information.
<div style='border:none;border-top:solid #B5C4DF 1.0pt;padding:3.0pt 0in 0in 0in'>

<p class=MsoNormal style='mso-outline-level:1'><span style='mso-bookmark:_MailOriginal'><b><span
style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'>From:</span></b></span><span
style='mso-bookmark:_MailOriginal'><span style='font-size:10.0pt;font-family:
"Tahoma","sans-serif"'> Scott R. Chamberlain <br>
<b>Sent:</b> Friday, October 18, 2013 9:30 AM<br>
<b>To:</b> REDACTED<br>
<b>Subject:</b> REDACTED<o:p></o:p></span></span></p>

</div>

