I believe you are being Joe-Jobbed

Update:

 

I reviewed the SMTP logs for the emails in question.  I also found several other read receipts that are being sent at the same time as the one listed above.  These receipts are from older messages (2 weeks - several months ago).  One of the users in our office has reported to me that she has been receiving an abundance of read receipts from old messages.  After some investigating, I found that all of our accounts were re-sending old receipts again and again.  We are using Outlook 2007, and I followed the instructions here (http://www.howto-outlook.com/howto/deletereadreceipt.htm) to clear out the "stuck" receipts.  Unfortunately, this did not resolve the issue.  

 

On a side note, we also lost IMAP folder access last week and this seems to coincide with the read receipt problem.  In order to fix the folder issue, we had to uncheck "When displaying hierarchy in Outlook, show only subscribed folders". That fixed everyone except for one user, who we switched over to Thunderbird.  This is when the read receipts started popping up.  

 

So, not sure if this is a Smartermail problem, or Outlook, or both.  Seriously considering changing everyone over to Thunderbird anyway.