Vertical line appears on left side of text window when writing e-mail

After I replied to an e-mail message a couple of days ago, now every time I am composing an outgoing message a vertical line appears on the left side of the text window. When I click on the "New" tab to create a message, an outline of the message box appears, but almost instantly the horizontal line at the top disappears, but the vertical line on the left stays. I sent a test message to myself and the line does not show up in the received message. So this is just an annoyance, but I would like to fix it. Please help with a solution.