I am experiencing the same problems. For me at least, the occurrence has nothing to do with receiving a spam message in the folder (I checked this out by receiving all emails, closing Outlook, repopening it and hitting the receive button to make sure that I did NOT receive a new spam message. Then I went to my "Sent" folder of emails. The subject line would automatically change if I (a) just clicked on the email message in the list of sent emails, (b) double clicked the email to open it up in Reading mode and (c) if I chose to "Forward" the message. On option "c" this also changed the subject line in the body of the email about one second after the subject line in the main part of the email was changed.
Also, I am using Westhost and have multiple domain email addresses through my Outlook accounts (i.e. comcast.net is one of them) No gmail, hotmail, yahoo accounts are coming through this Outlook account.