If you are using Outlook Express you can specify to have the greater than symbol (>), vertical bar (|), colon (:) or nothing precede each line of the forwarded message as follows:

1) From the main Outlook Express (OE) display open the "TOOLS" menu and select OPTIONS.
2) The OPTIONS multi-tab dialog box will appear, move to the "SEND" dialog and tab to the first set of RADIAL buttons and identify which format you are using (HTML or Plain Text). This is the "Mail Sending Format" segment of this dialog and is not evident if you are totally blind because JAWS does not read the "Mail Sending Format" header. But if you use the JAWS cursor (route it to the PC cursor) and then move it up you will see that this is indeed the "Mail Sending Format" section of the "Send" dialog. One additional hint, immediately upon landing on the "International Format" button, the next tab will land you on the HTML/Plain-Text radial buttons.
3) Now, tab to the appropriate "Settings" button (either HTML or Plain Text as you noted to be selected from the previous radial buttons) and select that button. NOTE: If you tab beyond the first pair of HTML/Plain-Text Settings buttons you will notice another such pair of buttons, but these are for news sending rather than mail sending.
4) Once you select the appropriate "Settings" button you will be placed into another dialog box. Tab around until you come to a check box that says "Indent Original Text With" and make sure this box is not checked. This will eliminate the annoying symbols that accompany many forwarded messages. NOTE: If you leave that check box checked you will then tab to a list where you can select the >, | or : symbols as the indentation character of choice.