There is no such thing like a message separator.

When looking at ML digest emails (i.e. from the IMP mailing list), IMP 
tries to assist by formatting the output (both in preview and message 
view).

If it encounters a signature below an individual message, it validly 
switches the displaying it in "light grey" (by starting a '<span 
class="signature">') - but it will only switch back (end the <span>) 
when encountering a quote.

--- cut here ---
<br>
I will see about filing a bug report with Debian about this.<br>
<br>
<span class="signature">
-- <br>
Andy Dorman<br>
<br>
<br>
<br>
------------------------------<br>
<br>
Message: 2<br>

[...]

<br>
Quoting Rick Romero &lt;<a href="#" 
onclick="void(window.open('/horde/imp/dynamic.php?page=compose&amp;to=rick%40havokmon.com&amp;popup=1','','width=820,height=610,status=1,scrollbars=yes,resizable=yes'));return false;">rick@havokmon.com</a>&gt;:<br></span><div class="citation 
quoted1">Hi,<br>
<br>
I have
--- cut here ---

It'd be nice (and easier to read :D ) if the message separator itself 
could be detected and would end the span... the next quote may be 
messages away.