I noticed that SalesForce adds this to the very bottom of my outbound HTML emails:
<br><img src="http://na8.salesforce.com/servlet/servlet.ImageServer?oid=11D4ABCDEFGH9Lo&esid=1234567890ABCDE"></body></html><br><br><DIV style="display:none;"></DIV>
(cluster farm, OID, and ESID have been renamed to protect the innocent :-) )
My templates are in plain-text (I occassionally switch to HTML so I can add things like indentation or bullet points). In any event though, I'm not explicitly adding the portion above. What is it for?