HTML CSS Field Layout
If the input with the placeholder="Email" is actually an email address, then the type attribute should be set to email, not text.  (Reference: http://www.w3.org/TR/html-markup/input.email.html)
Declare an id attribute and a name attribute on the email field.  This will be necessary if you want to access the elements or their posted values explicitly. (Reference: http://reference.sitepoint.com/html/input/name , http://reference.sitepoint.com/html/core-attributes/id)
Once you have declared an id on the input, set the for attribute on the label equal to that id.  This will enable a behavior such that clicking on the label will focus on the input for the corresponding field.  (Reference: http://reference.sitepoint.com/html/label)  
Use descriptive, semantic class names.  Good class names should indicate the nature of the content, not the format of the layout.  Instead of fieldwrapper try sender-email. (Reference : http://www.w3.org/QA/Tips/goodclassnames )
This depends on what HTML version you are using, but the HTML5 spec says that form elements should be wrapped in p tags, not divs.  (Reference: http://dev.w3.org/html5/spec/forms.html#writing-a-form's-user-interface)


The code that would correspond to my comments might look like : 

&lt;p class="email-input"&gt;
    &lt;label for="sender"&gt;Sender email address:&lt;/label&gt;
    &lt;input id="sender" name="sender" type="email" placeholder="Email" class="r2"&gt;
&lt;/p&gt;
&lt;p&gt;This is just me explaining how to use this particular field. It is very interesting, don't you know...&lt;/p&gt;


One thing you'll notice is that there are less CSS classes.  I have replaced the field-wrapper with email-input and completely removed the field class.  You can migrate the .field-wrapper CSS to the .email-input selector, and the field CSS to the .email-input &gt; input selector.