Those marks get there due to settings in the senders email client. For example, with Windows Mail (Windows Vista), this setting is shown in this screen print:
Outlook Express has the same setting.
I found a setting in Outlook 2007 that would enable the feature. Apparently, the default setting upon installation of Office is Disable. If you want to know where the setting is in Outlook, let me know. I have Outlook 2003 on another ccmputer.
Are they table or section borders?
I'm not sure of the exact technical classification, but I see I can remove them by going into the Source view in Windows Mail and removing some code lines.