Borders on forwarded email

As messages get forwarded, some computers place blue borders along the left
margin. How do these borders originate? One web source hinted this may be
aMac feature. Is there any way to clear them en masse?

They can be removed one at a time by placing the cursor inside a
border,selecting Format-->Borders and Shading-->then selecting a border
fromthe list ((this range is from 'table' and 'paragraph' to 'innermost'
to'outermost' and maybe a dozen between, many of which aren't evenvisible));
then clicking on the border if one appears in the previewpane and deleting it
there, then selecting OK and answering Yes in the next dialog box that
threatens possible corruption if you do and, if I'm lucky, the border
disappears. Many times there isn't any indication of a border in the Preview
box, so I'll "shoot in the dark" by making a border selection (usually
'outermost') and if the dialog box appears I can erase that border. If not,
I go back and try another border level and repeat these steps.

Programs like Email Stripper will do it, but it will also strip all the fonts
and images, leaving only plain text.