Extra characters in Email using System.Web.Mail?

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From: GSK (gsk_at_NiOcSaP.nAeMt)
Date: 06/02/04 

Date: Wed, 2 Jun 2004 09:00:46 -0400

I'm using System.Web.Mail to send HTML formatted e-mails, and something very
bizarre is happening.

When I send a message to myself, it looks fine, but when my client sends one
to herself, she gets a single exclamation point inserted, usually in the
same spot but sometimes it shifts a place or two, within the text.

The message is about 13 lines, and it is only one character that gets
inserted, so I find this very strange. There is no ASCII that could be
mis-represented in the source, and I have re-entered the text a couple of
times.

For example, the line:

message.Body = "valid for 30 days";

appears in her e-mail as:

valid f! or 30 days

It is getting embedded in the source as I see it in the messages she returns
to me.

Has anyone seen anything like this? I can only guess that her mail client
(Outlook) is embedding the character, but why?