I have been testing this today, and I have a small form that I am testing sending emails with.
First I render a small piece of HTML on a Rich Text Field, and then I transfer the raw HTML to a textarea to analyze it.

Something like:

<span>
<br>test1
<br>test2
<br>test3
<br>test4
</span>
The rendered HTML looks fine, and the code in the textarea look fine. When I email it though and Outlook receives it, every <br> is doubled, so the content is now: <span><br><br>test1<br><br>test2<br><br>test3<br><br>test4</span>

which of course renders with an extra line break. At first I thought it was outlook but the more I look into it, it seems that K2 sends it that way. Has anyone had this problem?